#include <cstdio>
#include <cmath>
using namespace std;
#define F(x) abs(x)>1e-6
int main(){
    double x,y,a,b,s,f;
    while(scanf("%lf%lf",&x,&y)&&(F(x+1.0)||F(y+1.0))){
        s=f=0;
        while(scanf("%lf%lf",&a,&b)&&(F(a)||F(b))){
            if(b<y){
                s+=a-x;f+=y-b;
            }
            x=a;y=b;
        }
        printf("%.0lf\n",s/f*y);
    }
}