#include <bits/stdc++.h>
using namespace std;

int main(){
	double Xa,Ya,Xb,Yb;
	cin >> Xa >> Ya >> Xb >> Yb;
	long double ans = Yb - (Yb - Ya) / (Xb + Xa) * Xb;
	printf("%.6Lf\n",ans);
	return 0;
}