#include <bits/stdc++.h>
using namespace std;
int di[] = {1, 0, -1, 0};
int dj[] = {0, 1, 0, -1};
int Di[] = {1, 1, 0, -1, -1, -1, 0, 1};
int Dj[] = {0, 1, 1, 1, 0, -1, -1, -1};
int main() {
  long double a, b, c, d;
  cin >> a >> b >> c >> d;
  long double ans = a / b;
  long double q = (1 - (a / b)) * (1 - (c / d));
  ans /= (1 - q);
  cout << fixed << setprecision(8) << ans;
  return 0;
}
