#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define EPS 1e-9
#define all(x) x.begin(),x.end()
#define allr(x) x.rbegin(),x.rend()
#define yes cout << "YES" << '\n';
#define no cout << "NO" << '\n';
#define mem(arrr, xx) memset(arrr,xx,sizeof arrr)
#define endl '\n'
#define PI acos(-1)
#define Ones(n) __builtin_popcount(n)
#define Onesl(n) __builtin_popcountll(n)
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
const ll MOD = 1e9 + 7;
const ll N = 2e5 + 5;
const int OO = 0x3f3f3f3f;
const ll LOO = 0x3f3f3f3f3f3f3f3f;
int dx8[] = {+0, +0, -1, +1, +1, +1, -1, -1};
int dy8[] = {-1, +1, +0, +0, +1, -1, +1, -1};
int dx4[] = {+0, +0, -1, +1};
int dy4[] = {-1, +1, +0, +0};

void Fast() {
    cin.tie(nullptr), cout.tie(nullptr), cin.sync_with_stdio(false), cout.sync_with_stdio(false);
#ifdef Clion
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}

void Solution() {
    int n;
    cin >> n;
    ld l = 0, r = 1e5;
    for (int i = 0; i < 100; ++i) {
        ld mid = (l + r)/2;
        ll sum = 0;
        for (ll j = 1; j <= mid; ++j) {
            sum += (ll)sqrtl(mid * mid - j * j);
        }
        sum *= 4;
        if(sum > n){
            r = mid;
        }
        else{
            l = mid;
        }
    }
    cout << setprecision(10) << fixed << r << '\n';
}

int main() {
    Fast();
    int T = 1;
//    cin >> T;
    while (T--) {
        Solution();
    }
}