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

int main()
{
    //freopen("input","r",stdin);

    double t1,t2,k2,k1;
    cin >> t1 >> k1 >> t2 >> k2;
    cout << fixed << setprecision(5) << t1*t2 / (k1*t2 + k2*t1);
}