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

signed main(){
  double xa, ya, xb, yb; cin >> xa >> ya >> xb >> yb;
  double dx = xb - ( -xa );
  double dy = yb - ya;
  dy *= xa / dx;
  cout << fixed << setprecision( 13 ) << ya + dy << endl;
  return 0;
}