2013寒假练习 1034：丈量土地

#include<iostream>
#include<cmath>
using namespace std;
typedef struct
{
double x,y;
void in()
{
scanf("%lf%lf",&x,&y);
}
}point;
double operator ^ (point a,point b)
{
return a.x*b.y-a.y*b.x;
}
int main()
{
int n,i;
point t1,t2,t;
while(~scanf("%d",&n))
{
double ans=0;
t1.in(),t=t1;
for(i=1;i<n;i++)
{
t2.in();
ans+=t1^t2;
t1=t2;
}
ans+=t1^t;
printf("%.2lf\n",fabs(0.5*ans));
}
return 0;
}