#include <iostream>
#include <cstdio>
int main() {
    double left_train_speed, right_train_speed, distance, bee_spped;

    std::cin >> left_train_speed >> right_train_speed;
    std::cin >> distance >> bee_spped;

    printf("%.10f\n", distance / (left_train_speed + right_train_speed) * bee_spped);

    return 0;
}