#include <bits/stdc++.h>

using namespace std;

long double t1,t2,k1,k2, T1, T2, S, R;

int main()
{
    cin >> t1 >> k1 >> t2 >> k2;
    T1 = t1/k1;
    T2 = t2/k2;
    cout << setprecision(5) << fixed << T1*T2/(T1 + T2);


    return 0;
}