#include <bits/stdc++.h>
typedef long long ll;
using namespace std;

bool check(ll x, ll s) {
	ll cnt = 0;
	for (ll i = 1; i * i <= x; ++i) {
		ll l = 0, r = sqrt(x);
		ll res = x - 1ll * i * i;
		while (l <= r) {
			ll mid = (l + r) >> 1;
			if (mid * mid <= res)
				l = mid + 1;
			else 
				r = mid - 1;
		}
		cnt += l - 1;//, cout << cnt << ',';
	}
	// cout << x << ':' << cnt << ',' << s << endl;
	return cnt < s;
}
int main() {
	ll s;
	cin >> s;
	s = (s + 3) / 4;
	// cout << s << endl;
	ll l = 0, r = 3 * s;
	while (l <= r) {
		ll mid = (l + r) / 2;
		if(check(mid, s))
			l = mid + 1;
		else 
			r = mid - 1;
	}
	// cout << l << endl;
	cout << fixed << setprecision(10) << sqrt(l) << '\n';
}