#include <bits/stdc++.h>
using namespace std;
double p1, p2;
double fastPow(double b, int n) {
  if (n == 0) return 1.0;
  double po = fastPow(b, n / 2);
  po = po * po;
  if (n & 1) return po * b;
  return po;
}
double proba(int turno) {
  double ans = 1;
  ans = ans * fastPow(1 - p1, turno / 2);
  ans = ans * fastPow(1 - p2, turno / 2);
  return ans * p1;
  return ans * p1;
}
int main() {
  double a, b, c, d;
  while (cin >> a >> b >> c >> d) {
    double prob = 0;
    p1 = a / b;
    p2 = c / d;
    for (int i = 1; i < 100000; i += 2) prob = prob + proba(i);
    cout << prob << endl;
  }
}
