//#include <fstream>
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <iomanip>

using namespace std;

//ifstream cin("tema.in");
//ofstream cout("tema.out");

int main() {
    double t1, t2, k1, k2;
    cin >> t1 >> k1 >> t2 >> k2;
    cout << fixed << setprecision(6) << t1 * t2 / (t1 * k2 + t2 * k1) << "\n";
    return 0;
}