#include <bits/stdc++.h>

int main() {

  double x1, y1;
  double x2, y2;

  std::cin >> x1 >> y1;
  std::cin >> x2 >> y2;

  double y;
  y = (y2 - y1) / (x1 + x2) * (x1) + y1;

  printf("%.20lf\n", y);

  return 0;
}