#include <iostream>
#include<iomanip>
using namespace std;

int main()
{

    double t1,t2, k1, k2;
    cin>>t1>>k1>>t2>>k2;

    double sol = (t1*t2)/(k1*t2+k2*t1);
    cout<<setprecision(8)<<sol;


    return 0;
}