#include<iostream>
#include<fstream>
using namespace std;
int main()
{
	double t1, t2, k1, k2;
	double distance = 1;
	cin >> t1;
	cin >> k1;
	cin >> t2;
	cin >> k2;
	double v1, v2;
	v1 = distance / t1;
	v2 = distance / t2;
	double result=(double)distance/(v1*k1 + v2*k2);
	cout << result;
	return 0;
}