#include <iostream>

using namespace std;
double t3;
int main()
{
    double v1, v2, t1, t2, k1, k2;

    cin>>t1>>t2;
    cin>>k1>>k2;

    t3=(t1*t2)/(t1*k2+t2*k1);
    cout<<t3;
    return 0;
}