#include <iostream>
#include <iomanip>

int main() {
    long vr, vl;
    long d;
    long w;
    double s;
    std::cin >> vr >> vl >> d >> w;

    s = (double)d / (vr + vl);

    std::cout << std::fixed << std::setprecision(7) <<w * s << std::endl;
}