#include <iostream>

using namespace std;

int k1 , k2 , t1 , t2;
int t;

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

    t = t1 * t2 * 10000 / (k1 * t2 + k2 * t1);

    cout << t / 10000 << "." << t % 10000;

}