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

int main() {
    int vl, vr, d, w;
    cin >> vl >> vr >> d >> w;

    printf("%.8f\n", (double)d * w / (vl + vr));
}