#include <iostream>
#include <cstdio>

using namespace std;

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

    double v1 = t2;
    double v2 = t1;

    double d = t1 * v1;

    double mytime = d / (v1 * k1 + v2 * k2);

    cout << mytime << '\n';

    return 0;
}