#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("date.in");


int main()
{
    int t1, t2, t3, k1, k2;
    double answer;
    cin >> t1 >> k1 >> t2 >> k2;
    //v1 = d/t1;
    //v2 = d/t2;
    //v1 * k1 + v2 * k2 = d / t3
    //k1/t1 + k2/t2 = 1/t3
    //t3 = 1/(k1/t1+k2/t2) = t1*t2/(k1*t2+k2*t1)
    answer = 1.0 * t1 * t2 / (k1 * t2 + k2 * t1);
    cout << setprecision(5) << fixed << answer;
    return 0;
}