/*
    /\_/\
   (= ._.)
   / >  \>
*/
#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
#include <cstdio>
#include <cmath>
#include <cassert>
#define int long long
#define double long double
using namespace std;
const int N = 4e5 + 10, M = 1e6 + 10, MOD = 1e9 + 7;
int a[N];
pair<int, int> b[N];
int check(double r) {
	int ans = 0;
	for (int i = 1; i <= r; i++) {
		int x = i, y = sqrt(r * r - x * x);
		ans += y;
	}
	return ans * 4;
}
void solve() {
	int n;
	cin >> n;
	double l = 0, r = 1e5;
	while (r - l > 1e-10) {
		double mid = (r + l) / 2;
		if (check(mid) >= n) {
			r = mid;
		} else {
			l = mid;
		}
	}
	printf("%.10Lf", l);
}
signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int TT = 1;
	//cin >> TT;
	while (TT--) {
		solve();
	}
	return 0;
}
/*
    /\_/\
   (= ._.)
   / >  \>
*/