#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    double t1, t2, k1, k2;
    cin >> t1 >> k1 >> t2 >> k2;
    cout << setprecision( 7 ) << fixed;
    cout << (t1 * t2) / (t1 * k2 + t2 * k1) << "\n";
    return 0;
}