#include <bits/stdc++.h>

using namespace std;

inline void debugMode() {
    #ifndef ONLINE_JUDGE
    freopen("debug.in", "r", stdin);
    #endif // ONLINE_JUDGE
}

int main()
{
    ios :: sync_with_stdio(false);
    debugMode();
    double kf,ks,tf,ts,t;


    cin >> tf >> kf >> ts >> ks;

    t  = (tf * ts) / (tf * ks + ts * kf);
    cout << t;

    return 0;
}