#include <iostream>
#include <iomanip>

using namespace std;

double t1, t2, k1, k2, d;

int main()
{
    cin >> t1 >> k1 >> t2 >> k2;

    cout << fixed << setprecision(5) << 1.0 / (k1 / t1 + k2 / t2);
    return 0;
}