#include <bits/stdc++.h>

#define pb push_back
#define f first
#define s second
#define pii pair<int, int>
#define mp make_pair
 
using namespace std;

int main() {
	cin.sync_with_stdio(false);

	long double t1, k1, t2, k2;
	cin >> t1 >> k1 >> t2 >> k2;

	cout << t1 / (k1 + (t1 / t2) * k2);

	return 0;
}