41 lines
708 B
C++
41 lines
708 B
C++
|
//G - <20>ĸﴺ<C4B8>紵<EFBFBD><E7B4B5><EFBFBD><EFBFBD>
|
|||
|
|
//https://vjudge.net/contest/508277#problem/G
|
|||
|
|
#include<iostream>
|
|||
|
|
#include<cstdio>
|
|||
|
|
#include<cmath>
|
|||
|
|
#include<cstring>
|
|||
|
|
using namespace std;
|
|||
|
|
const int N=101;
|
|||
|
|
double ans;
|
|||
|
|
int n,j;
|
|||
|
|
struct dot
|
|||
|
|
{
|
|||
|
|
int x,y;//int->double
|
|||
|
|
} a[N];
|
|||
|
|
double cross(double x1,double y1,double x2,double y2)
|
|||
|
|
{
|
|||
|
|
// printf("(%d,%d)(%d,%d):%d\n",x1,y1,x2,y2,x1*y2-x2*y1) ;
|
|||
|
|
return x1*y2-x2*y1;
|
|||
|
|
}
|
|||
|
|
int main()
|
|||
|
|
{
|
|||
|
|
while(1)
|
|||
|
|
{
|
|||
|
|
scanf("%d",&n);
|
|||
|
|
if(n==0) return 0;
|
|||
|
|
ans=0;
|
|||
|
|
memset(a,0,sizeof(dot)*(n+1));
|
|||
|
|
for(int i=1; i<=n; i++)
|
|||
|
|
{
|
|||
|
|
scanf("%d%d",&a[i].x,&a[i].y);
|
|||
|
|
}
|
|||
|
|
for(int i=1; i<=n; i++)
|
|||
|
|
{
|
|||
|
|
j=i%n+1;
|
|||
|
|
ans+=(cross(a[i].x,a[i].y,a[j].x,a[j].y)/2);
|
|||
|
|
}
|
|||
|
|
printf("%.1lf\n",ans);//Presentation Error
|
|||
|
|
}
|
|||
|
|
return 0;
|
|||
|
|
}
|