#include <bits/stdc++.h>
using namespace std;
int main(){
	int xa,xb,ya,yb;	cin>>xa>>ya>>xb>>yb;
	xa*=-1;
	int difx=xb-xa,dify=yb-ya;
	double py=(double)dify/difx*(-xa)+ya;
	printf("%.15lf\n",py);
	return 0;
}