#include <bits/stdc++.h>
using namespace std;
double a, b, c, d;
int main(void) {
  std::ios_base::sync_with_stdio(false);
  cin >> a >> b >> c >> d;
  double p = 0, p2 = 0;
  for (int i = 0; i < 1000000; i++) {
    p = p + (1 - p2 - p) * a / b;
    p2 = p2 + (1 - p2 - p) * c / d;
  }
  printf("%.9f", p);
}
