#include <bits/stdc++.h>
namespace Acc {
int a, b, c, d;
void work() {
  scanf("%d%d%d%d", &a, &b, &c, &d);
  printf("%.12f",
         (double)a * d / ((double)a * d + (double)b * c - (double)a * c));
}
}  // namespace Acc
int main() { return Acc::work(), 0; }
