#include <cstdio>
int main() {
	int x1,y1,x2,y2;
	scanf("%d %d",&x1,&y1);x1*=-1;
	scanf("%d %d",&x2,&y2);
	double a=(double)(y1-y2)/(x1-x2);
	printf("%.15lf\n",y2-a*x2);
}