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

float t1, t2, k1, k2;


int main()
{
    #ifndef ONLINE_JUDGE
    freopen("input", "r", stdin);
    freopen("output", "w", stdout);
    #endif // ONLINE_JUDGE

    cin >> t1 >> t2 >> k1 >> k2;
    cout << (t1*k2 + t2*k1)/(k1*k2);
    return 0;
}