hdu 2036

水题

AC代码：

#include<iostream>
using namespace std;
int main()
{
    float n,m,x,y,x1,y1,x2,y2;
    while(cin>>n,n){
        double sum=0.0;
    cin>>x>>y;
    x1=x,y1=y;
    n--;
    while(n--){
    cin>>x2>>y2;
    sum+=abs(x1*y2-x2*y1);
    x1=x2,y1=y2;}
    sum+=abs(x*y2-x2*y);
    printf("%.1f\n",sum/2);}
    return 0;
}