#include <stdio.h>
#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define ld long double
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pdd pair<ld, ld>
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second

int main() {
  cin.sync_with_stdio(false);

  int t1, k1, t2, k2;
  cin >> t1 >> k1 >> t2 >> k2;

  double sol = t1 * t2;
  sol /= (k1 * t2 + k2 * t1);
  cout << fixed << setprecision(6) << sol << '\n';

  return 0;
}