#include <bits/stdc++.h>

using namespace std;

int main()
{
    int t1, t2, k1, k2;
    int p, s;
    double t;
    cin >> t1 >> k1 >> t2 >> k2;
    p = t1*t2;
    s = t2*k1+t1*k2;
    t = (double)p/(double)s;
    cout << fixed << setprecision(4) << t << "\n";
}