#include <bits/stdc++.h>
using namespace std;
using uint = unsigned int;
using ll = long long;
using pii = pair<int, int>;
#define dbg(x) cerr<<#x": "<<(x)<<'\n'
#define dbg_v(x, n) cerr<<#x"[]: ";for(long long _=0;_<n;++_)cerr<<(x)[_]<<' ';cerr<<'\n'
#define all(v) v.begin(), v.end()
#define NMAX 

int main()
{
#ifndef ONLINE_JUDGE
	ifstream cin("data.in");
	ofstream cout("data.out");
#endif
	ios_base::sync_with_stdio(false);

	double t1, t2, k1, k2;

	cin >> t1 >> k1 >> t2 >> k2;

	cout << 1.0 / (k1 / t1 + k2 / t2) << '\n';

	return 0;
}