#include <bits/stdc++.h>
using namespace std;
template <class t>
inline void rd(t &s) {
  s = 0;
  register char c = getchar();
  while (!isdigit(c)) c = getchar();
  while (isdigit(c)) s = (s << 3) + (s << 1) + (c ^ 48), c = getchar();
  return;
}
signed main(void) {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  double A = (double)a / (double)b, B = (double)c / (double)d;
  printf("%.9lf\n", A * (1.0 / (1.0 - (1.0 - A) * (1.0 - B))));
  return 0;
}
