#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  scanf("%d %d %d %d", &a, &b, &c, &d);
  double ans = ((double)(a * d) / (double)((b * c) + (a * d) - (a * c)));
  printf("%.12lf\n", ans);
  return 0;
}
