hdu2036（改革春风吹满地）

#include <iostream>

#include <cstdio>

#include <cstring>

#include <algorithm>

#include <vector>

using namespace std;

const int maxn = 110;



struct Point{

    double x, y;

    Point(double x = 0, double y = 0):x(x), y(y) { }

};



typedef Point Vector;

Vector v[maxn];



double Cross(Vector A, Vector B) {

    return A.x * B.y - A.y * B.x;

}



Vector operator - (Point A, Point B) {

    return Vector(A.x-B.x, A.y-B.y);

}



double getArea(Point* p, int n) {

    double area = 0;

    for(int i = 1; i < n-1; i++) {

        area += Cross(p[i]-p[0], p[i+1]-p[0]);

    }

    return area/2;

}



int main()

{

    int n;

    int x, y;

    while(scanf("%d", &n)!=EOF) {

        if(n==0) break;

        for(int i = 0; i < n; i++){

            scanf("%d%d", &x, &y);

            v[i] = Point(x, y);

        }

        double res = getArea(v, n);

        printf("%.1lf\n", res);

    }

    return 0;

}