#include<bits/stdc++.h>
using namespace std;
const double eps = 1e-9;
long long get(double s)
{
    long long ans = 0;
    double p = s * s;
    for(int i = 1;i <= s;i++) {
        int d = ((int)(sqrt(p - i*i) + eps)) ;
        while(1LL*(d+1)*(d+1) + 1LL*i*i <= p) d++ ;
        while(1LL*d*d + 1LL*i*i > p) d--;
        ans += 4LL * d;
    }
    return ans;
}
int main()
{
    int s;cin >> s;
    double l = 1 , r = sqrt(s) + 20;
    while((r - l) / l > eps) {
        double md = (l + r) / 2;
        if(get(md) > s) r = md;
        else l = md;
    }
    printf("%.12lf\n",(l + r) / 2);
    return 0;
}