#include <bits/stdc++.h>
using namespace std;
int main() {
  float q, w, e, t;
  cin >> q >> w >> e >> t;
  float a = q / w;
  float r = (1 - q / w) * (1 - e / t);
  float ans = a / (1 - r);
  printf("%.7f", ans);
  return 0;
}
