#include <bits/stdc++.h>
using namespace std;
int main() {
  int a1, b1, c1, d1;
  cin >> a1 >> b1 >> c1 >> d1;
  float a, b, c, d;
  a = (float)a1;
  b = (float)b1;
  c = (float)c1;
  d = (float)d1;
  float e;
  e = (a * d) / (b * d - (b - a) * (d - c));
  printf("%.7f", e);
  return 0;
}
