#include <iostream>
#include <iomanip>
using namespace std;

int main(void){
  int xa, ya;
  int xb, yb;
  double yp;
  cin >> xa >> ya >> xb >> yb;
  yp = -1.0 * xb * (yb - ya) / (xb + xa) + yb;
  cout << setprecision(9) << yp << endl;
  return 0;
}