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