#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;

int t1, t2, k1, k2;

int main()
{
	cin >> t1 >> k1 >> t2 >> k2;

	double res = (double)k1 / (double)t1 + (double)k2 / (double)t2;
	res = 1.0/(double)res;
	cout << res;

    return 0;
}