#include <bits/stdc++.h>

using namespace std ;

int main()
{
	//freopen ("input", "r", stdin) ;
	double t1, k1, t2, k2 ; 
	cin >> t1 >> k1 >> t2 >> k2 ; 
	double n1 = t1 * t2 ;
	double n2 = t2 * k1 + t1 * k2 ; 
	double sol = n1 / n2 ;
	cout << fixed << setprecision (5) << sol << '\n' ;
	return 0 ; 
}