#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using namespace std;

int main() {
    double vl, vr, d, w;
    cin >> vl >> vr >> d >> w;
    
    double ans = d/(vl+vr)*w;
    printf("%.6f\n", ans);
    
    return 0;
}