#include <bits/stdc++.h>
double abcxx00;
using namespace std;
const int maxn = 5 + 1000;
int main() {
  double a, b, c, d;
  while (cin >> a >> b >> c >> d)
    printf("%.15lf\n", a * d / (b * c - a * c + a * d));
  return 0;
}
