2036 改革春风吹满地

 

/*    s=1/2(            (x1*y2-x2*y1)+            (x2*y3-x3*y2)+            (xn-1*yn-xn*yn-1)+            ......            (xn*y1-x1*yn)         )*/#include<iostream>#include<iomanip>using namespace std;int main(void){    int n;    int x0,y0,x,y;    int tempx,tempy;    double area;    while(cin>>n&&n)    {        area=0;        cin>>x0>>y0;        tempx=x0;        tempy=y0;        while(--n)        {            cin>>x>>y;            area+=(x0*y-x*y0);            x0=x;            y0=y;        }        area+=x*tempy-y*tempx;        cout<<fixed<<setprecision(1)<<area/2<<endl;    }    return 0;}