#include <bits/stdc++.h>
using namespace std;
void test() {
  cout << fixed << setprecision(10);
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double A = a / b;
  double r = (1 - a / b) * (1 - c / d);
  double ans = A / (1 - r);
  cout << ans;
}
signed main() {
  ios::sync_with_stdio(false);
  long long testcases = 1;
  while (testcases-- > 0) {
    test();
  }
}
