#include"iostream.h"
double    calarea(double *x,double *y,int N)
{ double sum=0;
    for(int i=0;i<N;i++)
    {if(i<N-1)
             sum+=x[i]*y[i+1]-x[i+1]*y[i];
    else 
          sum+=x[i]*y[0]-x[0]*y[i];
    cout<<"sum中间数据："<<sum<<endl;
    }
    return sum/2.0;
}
void main()
{ int N,i;
cout<<"请输入你要计算多边形的顶点个数"<<endl;
cin>>N;
if(N>100)
{cout<<"你输入的N值超限，请重新输入"<<endl;
cin>>N;
}
double x[100];
double y[100];
cout<<"请分别输入你用于计算的坐标（x y）"<<endl;
for(i=0;i<N;i++)
{cout<<"请输入第"<<i+1<<"个坐标"<<endl;
cin>>x[i]>>y[i];
} 
cout<<"你输入的坐标是这样的："<<endl;
for(i=0;i<N;i++)
cout<<x[i]<<"    "<<y[i]<<endl;
cout<<"您的最终面积数据是："<<calarea(x,y,N)<<endl;
}