#include <bits/stdc++.h>

#define PAUSE system("pause")
#define LLI long long int
#define LD long double
#define PB push_back
#define PF push_front
#define MP make_pair
#define FORi(i, a, b) for(int i = a; i < b ; ++i)
#define FORd(i, a, b) for(int i = a; i > b ; --i)

using namespace std;

int main() {
	double t1,k1,t2,k2;
	cin >> t1 >> k1 >> t2 >> k2;
	cout << setprecision(5) << fixed << 1/(k1/t1+k2/t2);
	return 0;
}