#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long a, b, c, d;
  cin >> a >> b >> c >> d;
  float x = (float)a / b;
  float y = (float)c / d;
  float z = (1 - x) * (1 - y);
  cout << x / (1 - z) << endl;
}
int32_t main() {
  std::ios::sync_with_stdio(false);
  long long t;
  t = 1;
  while (t--) solve();
}
