#include <iostream>
#include <algorithm>
#include <cstring>


using namespace std;
#define ld long double
ld t1,t2,k1,k2;
int main()
{
    cin>>t1>>k1>>t2>>k2;

    cout<< 1/(k1/t1 + k2/t2)<<'\n';


    return 0;
}