hdu-1115-Lifting the Stone

#include<cstdio>
#include<cmath>
#include<iostream>
using namespace std;
int main()
{

    int s,n,i;
    double x1,x2,y1,y2,a,b;
    cin>>s;
    while(s--)
    {

        double x3,y3,area=0.0,area1=0.0,sumx=0.0,sumy=0.0;
        cin>>n;
        cin>>x1>>y1;
        cin>>x2>>y2;
        for(i=2;i<n;i++)
        {
            cin>>a>>b;
            x3=(x1+x2+a);
            y3=(y1+y2+b);
            area1=((x2-x1)*(b-y1)-(a-x1)*(y2-y1))/2;
            area+=area1;
            sumx+=x3*area1;
            sumy+=y3*area1;
            x2=a;
            y2=b;
        }
        printf("%.2lf %.2lf\n",sumx/area/3,sumy/area/3);
    }
    return 0;
}