#include <bits/stdc++.h>
int main() {
  int a, b, c, d;
  double x, y;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  x = (a * 1.0 / b);
  y = (1 - (a * 1.0 / b)) * (1 - (c * 1.0 / d));
  printf("%.12f\n", x * 1.0 / (1 - y));
  return 0;
}
