#include <bits/stdc++.h>

using namespace std;

double t1, t2, k1, k2, res;

int main() {
    cin >> t1 >> k1 >> t2 >> k2;

    res = t1 * t2 / (k1 * t2 + k2 * t1);

    cout << fixed << setprecision(5) << res << '\n';
    return 0;
}