sicily 1179. Extrusion

#include<iostream>        //计算多边形面积(利用向量叉乘)
#include<stdio.h>
#include<cmath>
using namespace std;
struct Point 
{ 
    double x, y;
}p[10000];
double polygon_area(int n)
{   
     double area=0;    
    for(int i=0;i<n-1;++i)
    {
        area+=p[i].x*p[i+1].y-p[i].y*p[i+1].x;
    }
    area+=p[n-1].x*p[0].y-p[n-1].y*p[0].x;    
    return fabs(area)/2;   
}
int main()
{
    int n;
    while(cin>>n,n>=3)
    {
        for(int i=0;i<n;++i)
            cin>>p[i].x>>p[i].y;
        double v;
        cin>>v;
        printf("BAR LENGTH: %.2f\n",v/polygon_area(n));
    }
    return 0;
}