#include <bits/stdc++.h>
using namespace std;
using ld = long double;

ld xa, ya, xb, yb;

int main(){
  cin >> xa >> ya >> xb >> yb;

  double ans = yb-(yb-ya)/(xb+xa)*xb;

  cout<<fixed<<setprecision(16);
  cout << ans << endl;
}