#include <bits/stdc++.h>
using namespace std;
double a, b, c, d;
int main() {
  while (cin >> a >> b >> c >> d) {
    double aux1 = (1 - (double)(a / b)), aux2 = (1 - (double)(c / d));
    printf("%.16lf\n", (double)(a / b) * (double)(1 / (1 - (aux1 * aux2))));
  }
}
