#include <bits/stdc++.h>
int main(int argc, char *argv[]) {
  int a, b, c, d;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  int numerator = a, denominator = b;
  numerator *= b * d;
  denominator *= (b * d - (b - a) * (d - c));
  printf("%.12lf\n", (double)(numerator) / (double)(denominator));
  return 0;
}
