#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  scanf("%d %d %d %d", &a, &b, &c, &d);
  int num = a * d;
  int den = b * c + a * d - a * c;
  printf("%.8lf\n", (double)num / (double)den);
  return 0;
}
