#include <bits/stdc++.h>

using namespace std;

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)

using ll = long long;
using P = std::tuple<int,int>;

const int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1}, dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    double vl, vr, d, w;
    std::cin >> vl >> vr >> d >> w;

    double pro = (w + vr) * (w + vl);
    
    printf("%.7f\n", 2. * w * w / pro / (1 - (w - vl) * (w - vr) / pro) * d);
}