#include <bits/stdc++.h>

using namespace std;
typedef double f64;

long long calc(f64 r) {
    long long res = 0;
    const f64 r2 = r * r;
    for (int l = 1; l < r; ) {
        int cnt = floor(sqrt(r2 - l * l));
        int nxt = floor(sqrt(r2 - cnt * cnt));
        res += 4ll * (nxt - l + 1) * cnt;
        l = nxt + 1;
    }
    return res;
}

int main() {
    int s;
    scanf("%d", &s);
    double l = 0, r = 1e9;
    while (r - l > 1e-7) {
        f64 mid = (l + r) / 2;
        if (calc(mid) < s) l = mid;
        else r = mid;
    }
    printf("%.10lf\n", l);
    return 0;
}