求多边形面积

#include <stdio.h>
#include<stdlib.h>
typedef struct point
{
int x,y;
}point;
point a[100];
double area(point p,point q)
{
return p.x * q.y - q.x * p.y;
}
int main()
{
int i,n;
double sum;
while(scanf("%d ",&n) != EOF && n)
{
for(i=0;i<n;i++)
scanf("%d %d",&a[i].x,&a[i].y);
sum = area(a[n-1],a[0]);
for(i=1;i<n;i++)
sum +=area(a[i-1] ,a[i]);
printf("%.1f\n",0.5 * sum);
}
system("pause");
return 0;
}