#include <bits/stdc++.h>

using namespace std;

const int NMAX = 750;

int v[NMAX+2];
int d[NMAX+2];
int N, Ans = 0;

int main()
{
    double T1, T2, K1, K2;
    cin >> T1 >> K1 >> T2 >> K2;
    double ans = T1*T2 / ( K1*T2 + K2*T1 );
    cout << setprecision(5) << fixed << ans << '\n';
    return 0;
}