利用坐标求任意多边形面积

#include <stdio.h>
int main ()
{
    int i,j,n;
    int x[100],y[100];
    while (scanf ("%d",&n),n)
    {
         float sum=0.0;
    for (i=0;i<n;i++)
    {
        scanf ("%d",&x[i]);
        scanf ("%d",&y[i]);
    }
    for (i=0,j=i+1;i<n,j<n;i++,j++)
        {
            sum+=(x[i]*y[j]-x[j]*y[i]);
        }
    sum+=(x[n-1]*y[0]-y[n-1]*x[0]);
        printf ("%.1f\n",sum/2.0);
    }
    return 0;
}