#include <iostream>
#include <iomanip>

using ldouble = long double;

void solve() {
    ldouble vl, vr, d, w;
    std::cin >> vl >> vr >> d >> w;

    std::cout << std::fixed << std::setprecision(10)
              << (w * d) / (vl + vr) << std::endl;
}

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

    solve();

    return 0;
}