#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int M = 1001;
const int mod = 7340033;
const double eps = 1e-6;
const double Pi = 2 * acos(0.0);
inline int max(int a, int b) { return a > b ? a : b; }
inline int min(int a, int b) { return a < b ? a : b; }
inline int bit(int x) { return 1 << x; }
inline double dabs(double x) { return x > 0 ? x : (-x); }
inline int abs(int x) { return x > 0 ? x : (-x); }
inline int lowbit(int x) { return x & (-x); }
int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  double p = 1.0 * a / b, q = 1.0 * c / d;
  cout << p / (1 - (1 - p) * (1 - q)) << endl;
  return 0;
}
