#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int num = a * d;
  int denom = ((a * d) + (b * c) - (a * c));
  double ans = (double)((double)num / (double)denom);
  cout << ans << endl;
  return 0;
}
