#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef double db;

ll mySqrt(ll num) {
    ll le = 0.5 * sqrt(num), ri = ceil(2 * sqrt(num)), ans = -1;
    while (le <= ri) {
        ll mid = (le + ri) >> 1;
        if (mid * mid <= num) {
            ans = mid;
            le = mid + 1;
        } else {
            ri = mid - 1;
        }
    }
    assert(ans >= 0);
    return ans;
}

ll check(ll r2) {
    ll res = 0;
    for (ll le = 1; le * le <= r2;) {
        ll cnt = mySqrt(r2 - le * le);
        ll ri = mySqrt(r2 - cnt * cnt);
        res += (ri - le + 1) * cnt * 4;
        le = ri + 1;
    }
    return res;
}

int main(void) {
    ll s;
    scanf("%lld", &s);
    ll le = 0, ri = 1e9, ans = -1;
    while (le <= ri) {
        ll mid = (le + ri) >> 1;
        if (check(mid) > s) {
            ans = mid;
            ri = mid - 1;
        } else {
            le = mid + 1;
        }
    }
    printf("%.10f\n", sqrt(ans));
    return 0;
}