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

double t1, k1, t2, k2, d;

int main() {

    cin >> t1 >> k1 >> t2 >> k2;

    d = 200;
    double v1 = d/t1;
    double v2 = d/t2;

    v1 *= k1;
    v2 *= k2;

    cout << fixed << setprecision(5) << d/(v1 + v2);

    return 0;
}