#include <bits/stdc++.h>
using namespace std;
void test() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  double num = a * d;
  double den = b * d - (b - a) * (d - c);
  num = num / den;
  cout << fixed << setprecision(7) << num << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    test();
  }
}
