#include <bits/stdc++.h>

using namespace std;

int main() {
	

	double d = 1;
	double t1, k1, t2, k2;
	cin >> t1 >> k1 >> t2 >> k2;

	double v = k1 / t1 + k2 / t2;
	cout << fixed << setprecision(12) << 1.0 / v << endl;

	return 0;
}