hdu 1392凸包周长

//用的自己的计算几何模板，不过比较慢嘿嘿
//要注意只有一个点和两个点
//Computational Geometry
//by kevin_samuel(fenice) Soochow University
//temple
#include <iostream>
#include <cmath>
#include <algorithm>
#include <cstdio>
//#include <stack>

using namespace std;

//define
const double EPS = 1e-8;
const double PI = acos(-1.0);



//point
//====================================================================
class Point
{
public:
  double x;
  double y;
  Point(){}
  Point(double x,double y):x(x),y(y){}
 
  //operator
  //operator=
  Point& operator=(const Point& _P)
  {
    x = _P.x;
    y = _P.y;
    return *this;
  }
  //operator*
  double operator*(const Point& _P)const
  {
    return x*_P.y - y *_P.x;
  }
  //operator-
  Point operator-(const Point& _P)const
  {
    return Point(x - _P.x,y - _P.y);
  }
  //operator==
  bool operator==(const Point& _P)const
  {
    if(fabs(_P.x - x) < EPS && fabs(_P.y - y) < EPS)
      return true;
    else
      return false;
  }
  bool operator!=(const Point& _P)const
  {
    return ((*this) == _P) == false;
  }
  
  //dot product
  static double dotProduct(Point s1,Point e1,Point s2,Point e2)
  {
    double result = 0;
    double x1 = e1.x - s1.x;
    double y1 = e1.y - s1.y;
    double x2 = e2.x - s2.x;
    double y2 = e2.y - s2.y;
    
    result = x1*x2 + y1*y2;
    return result;
  }

  //cross product 1 (4 point-type params)
  static double crossProduct(Point s1,Point e1,Point s2,Point e2)
  {
    double result = 0;
    double x1 = e1.x - s1.x;
    double y1 = e1.y - s1.y;
    double x2 = e2.x - s2.x;
    double y2 = e2.y - s2.y;
    
    result = x1*y2 - x2*y1;

    return result;
  }
  
  //cross product 2 (3 point-type params) 
  static double crossProduct(Point p1,Point p2,Point p0)
  {
    return crossProduct(p0,p1,p0,p2);
  }
  
  //Is on segment or line
  static bool onSegment(Point Pi,Point Pj,Point Q)
  {
    if(Q.x >= min(Pi.x,Pj.x) && Q.x <= max(Pi.x,Pj.x) &&
       Q.y >= min(Pi.y,Pj.y) && Q.y <= max(Pi.y,Pj.y) &&
       fabs(crossProduct(Q,Pj,Pi)) < EPS
     )
      return true;
    else
      return false;
  }
  
  //Is on segment
  bool IsOnSegment(Point Pi,Point Pj)
  {
    if(this->x >= min(Pi.x,Pj.x) && this->x <= max(Pi.x,Pj.x) &&
       this->y >= min(Pi.y,Pj.y) && this->y <= max(Pi.y,Pj.y) &&
       fabs(Point::crossProduct(*this,Pj,Pi)) < EPS
     )
      return true;
    else
      return false;
  }
  
  //Is inside triangle
  bool inTriangle(Point A,Point B,Point C)
  {
    
    double Sabc = fabs(Point::crossProduct(B,C,A));
    double Spab = fabs(Point::crossProduct(A,B,(*this)));
    double Spac = fabs(Point::crossProduct(A,C,(*this)));
    double Spbc = fabs(Point::crossProduct(B,C,(*this)));
    
    if(Spbc + Spab + Spac == Sabc)
      return true;
    else
      return false;
  }
  
  //Is inside polygon
  //polys[] ,0-n
  bool insidePolygon(Point *polys,int n)
  {
    int counter = 0;
    double xinters;
    Point p1,p2;
    p1 = polys[0];
    for(int i = 1; i < n; i++)
      {
	p2 = polys[i % n];
	if(Point::onSegment(p1,p2,(*this)) == true)
	  return true;
	if((*this).y > min(p1.y,p2.y) && (*this).y <= max(p1.y,p2.y))
	  {
	    if((*this).x <= max(p1.x,p2.x) && p1.y != p2.y)
	      {
		xinters = ((*this).y - p1.y)*(p2.x - p1.x)/(p2.y - p1.y) + p1.x;
		if(p1.x == p2.x || (*this).x <= xinters)
		  counter++;
	      }
	  }
	p1 = p2;
      }
    if(counter % 2 == 0)
      return false;
    return true;
  }
  
  //distance^2
  double dis2(const Point& _P)const
  {
    return (x - _P.x)*(x - _P.x) + (y - _P.y)*(y - _P.y);
  }
  //distance 
  double dis(const Point& _P)const
  {
    return sqrt(dis2(_P));
  }
    static double dis(const Point& p1,const Point& p2)
    {
        return sqrt((p2.x - p1.x)*(p2.x - p1.x) + (p2.y - p1.y)*(p2.y -p1.y));
    }
    
};

//==================================================================================

//vector segment or line
//==================================================================================

class Vector
{
public:
  Point start;
  Point end;
  double _x;
  double _y;
  double a,b,c;        //ax + by + c = 0
  

  Vector(){}
  Vector(double __x,double __y):_x(__x),_y(__y){}
  Vector(Point a,Point b):start(a),end(b) { _x = end.x - start.x; _y = end.y - start.y; }

  //operator
  //judge the position of the point relative to the segment
  double operator*(const Point& _P)const
  {
    double res = Point::crossProduct(_P,this->end,this->start);
    if(fabs(res) < EPS)
      return 0;
    if(res > 0)
      return 1;
    else
      return -1;
  }
  
  //crossProduct
  double operator*(const Vector& _V)const
  {
    Point st = _V.start;
    Point en = _V.end;
    return Point::crossProduct(start,end,st,en);
  }
  
  //transfer to normal rex
  bool pton()
  {
    a = start.y - end.y;
    b = end.x - start.x;
    c = start.x * end.y - start.y * end.x;
    return true;
  }

  //judge whether _P is on the line
  bool IsLinehasPoint(const Point& _P)const
  {
    if(fabs((*this)*_P) < EPS)
      return true;
    else
      return false;
  }
  
  //juegde whether _P is on the segment
  bool IsSegmenthasPoint(const Point& _P)const
  {
    if(_P.x >= min(this->start.x,this->end.x) && _P.x <= max(this->start.x,this->end.x) &&
       _P.y >= min(this->start.y,this->end.y) && _P.y <= max(this->start.y,this->end.y) &&
       fabs((*this)*_P) < EPS
    )
      return true;
    else
      return false;
  }

  //the distance from point to line
  double disToPoint(const Point& _P)
  {
    pton();  //transfer
    
    double td = (a*_P.x + b*_P.y + c)/sqrt(a*a + b*b);
    
    double xp = (b*b*_P.x - a*b*_P.y -a*c)/(a*a + b*b);
    double yp = (-a*b*_P.x + a*a*_P.y - b*c)/(a*a + b*b);
    double xb = max(start.x,end.x);
    double yb = max(start.y,end.y);
    double xs = start.x + end.x - xb;
    double ys = start.y + end.y - yb;
    
    if(xp > xb + EPS||xp < xs - EPS||yp > yb + EPS||yp < ys - EPS)
      td = min(_P.dis(start),_P.dis(end));
    
    return fabs(td);
  }

  //out the mirror point by this line or segment
  Point mirrorPoint(const Point& _P)const
  {
    Point ret;
    
    double d = a * a + b * b;
    ret.x = (b*b*_P.x - a*a*_P.x - 2*a*b*_P.y - 2*a*c)/d;
    ret.y = (a*a*_P.y - b*b*_P.y - 2*a*b*_P.x - 2*b*c)/d;
    
    return ret;
  }
  
  //out the vertical line of two points
  static Vector verticalLine(const Point& _a,const Point& _b)
  {
    Vector ret;
    ret.start.x = (_a.x + _b.x)/2;
    ret.start.y = (_a.y + _b.y)/2;
    
    ret.a = _b.x - _a.x;
    ret.b = _b.y - _a.y;
    ret.c = (_a.y - _b.y)*ret.start.y + (_a.x - _b.x)*ret.start.x;
    
    if(fabs(ret.a) > EPS)
      {
          ret.end.y = 0.0;
          ret.end.x = -ret.c/ret.a;
          if(ret.end == ret.start)
          {
              ret.end.y = 1e10;
              ret.end.x = -(ret.c - ret.b * ret.end.y)/ret.a;
          }
      }
    else
      {
          ret.end.x = 0.0;
          ret.end.y = - ret.c/ret.b;
          if(ret.end == ret.start)
          {
              ret.end.x = 1e10;
              ret.end.y = - (ret.c - ret.a*ret.end.x)/ret.b;
          }
      }
    return ret;
  }
  
  //line with line
  //two lines coincide
  bool equal(const Vector& _P)const
  {
    return IsLinehasPoint(_P.start) && IsLinehasPoint(_P.end);
  }

  // two parallel lines
  bool parallel(const Vector& _P)const
  {
    return (fabs((*this)*_P)) < EPS;
  }

  //the intersection points of two lines
  Point Intersection(Vector _V)
  {
    //judge parallel and coincide
    Point ret = start;
    double t = ((start.x - _V.start.x)*(_V.start.y - _V.end.y) - (start.y - _V.start.y)*(_V.start.x - _V.end.x))/
      ((start.x - end.x)*(_V.start.y - _V.end.y) - (start.y - end.y)*(_V.start.x - _V.end.x));

    ret.x += (end.x - start.x)*t;
    ret.y += (end.y - start.y)*t;
    return ret;
  }
  

  //line and segment
  //is line cross with segment
  //param:segment
  bool crossSL(const Vector& _V)const
  {
    double rs = (*this)*_V.start;
    double re = (*this)*_V.end;
    return rs*re < EPS;
  }

  //segment and segment
  //judge wheather two segments intersect
  bool IsCrossSS(const Vector& _V)const
  {
    return (
	    max(start.x,end.x) >= min(_V.start.x,_V.end.x) &&
	    max(_V.start.x,_V.end.x) >= min(start.x,end.x) &&
	    max(start.y,end.y) >= min(_V.start.y,_V.end.y) &&
	    max(_V.start.y,_V.end.y) >= min(start.y,end.y) &&
	    ((Vector(_V.start,start)*_V)*(_V*Vector(_V.start,end)) >= EPS) &&
	    ((Vector(start,_V.start)*(*this))*((*this)*Vector(start,_V.start))>=EPS)

	    );
  }
  
  
};
//=================================================================================
//Graham-scan
#define MAXN 110
Point Points[MAXN];

int N;

int stack[MAXN];
int top;

bool cmp(const Point& a,const Point& b)
{
  if(a.y == b.y)
    return a.x < b.x;
  else
    return a.y < b.y;
}

bool multi(Point sp,Point ep,Point op)
{
    return (sp.x - op.x)*(ep.y - op.y) >= (ep.x - op.x)*(sp.y - op.y);
}

void Graham_Scan()
{
    int len;
    top = 1;
    sort(Points,Points+N,cmp);
    if(N == 0)
        return;
    stack[0] = 0;
    if(N == 1)
        return;
    stack[1] = 1;
    if(N == 2)
        return;
    stack[2] = 2;
    for(int i = 2; i < N; i++)
    {
        while(top && multi(Points[i],Points[stack[top]],Points[stack[top-1]])) top--;
        stack[++top] = i;
    }
    len = top;
    stack[++top] = N - 2;
    for(int i = N - 3; i >= 0; i--)
    {
        while(top != len && multi(Points[i],Points[stack[top]],Points[stack[top-1]])) top--;
        stack[++top] = i;
    }
}
//==================================================================================
//test zone

int main()
{
    while(cin>>N)
    {
        if(N == 0)
            break;
        for(int i = 0; i < N; i++)
        {
            cin>>Points[i].x>>Points[i].y;
        }
        Graham_Scan();
        double length = 0;
        for(int i = 0; i <= top - 2; i++)
        {
            length +=Point::dis(Points[stack[i]],Points[stack[i+1]]);
        }
        length +=Point::dis(Points[stack[top - 1]],Points[stack[0]]);
        if(N == 1)
            printf("0.00\n");
        else if(N == 2)
            printf("%.2lf\n",Point::dis(Points[0],Points[1]));
        else
            printf("%.2lf\n",length);
    }
  return 0;
}