#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d, res, x, y, z, p;
  cin >> a >> b >> c >> d;
  x = 1 - (a / b);
  y = 1 - (c / d);
  p = 1 - (x * y);
  z = (1 / p);
  res = (1 - x) * z;
  cout << setprecision(10) << res;
  return 0;
}
