#include <bits/stdc++.h>
using namespace std;
const long long inf = 1 << 28;
double a, b, c, d, space;
int main() {
  while (cin >> a >> b >> c >> d) {
    a = a / b;
    c = c / d;
    space = a * c + a * (1 - c) + (1 - a) * c;
    printf("%.12lf\n", a / space);
  }
  return 0;
}
