#include <bits/stdc++.h>
using namespace std;

int main() {

    int t1, k1, t2, k2; cin >> t1 >> k1 >> t2 >> k2;
    double v1 = 1.0 / (t1 * 1.0);
    double v2 = 1.0 / (t2 * 1.0);

    v1 *= k1;
    v2 *= k2;

    cout << fixed << setprecision(10) << 1.0 / (v1 + v2) << "\n";

    return 0;
}