HDU 2036 改革春风吹满地

求面积~

不用考虑凹凸多边。

#include <stdio.h>

int main()
{
    int n;
    int x1,y1,x2,y2,x3,y3,i;
    double ans;
    while(scanf("%d",&n)!=EOF&&n)
    {
        //cin>>x1>>y1>>x2>>y2;
        scanf("%d%d%d%d",&x1,&y1,&x2,&y2);
        ans = 0;
        for(i = 2;i < n;i ++)
        {
            scanf("%d%d",&x3,&y3);
            ans += double((x2-x1)*(y3-y1)-(y2-y1)*(x3-x1));
            x2 = x3;y2 = y3;
        }
        printf("%.1llf\n",ans/2);
    }
}