#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  double x = a * d;
  double y = b * d - (b - a) * (d - c);
  printf("%.6f\n", x / y);
  return 0;
}
