/*
    /\_/\
   (= ._.)
   / >  \>
*/
#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;
}