BINHPHUONG C++

#include <bits/stdc++.h>
using namespace std;
#define ll long long
ll k;
ll dem(ll s)
{
    ll d=0;
    for(ll i=1;i<=(ll)sqrt(s);i++)
    {
       d += (ll)    (sqrt(s-i*i));
    }
    return d;
}
void giai()
{
    ll l=2,r=1e12,mid,kq;
    while(l<=r)
    {
        mid=(l+r)/2;
        if(dem(mid)>=k)
        {
            kq =mid;
            r=mid-1;
        } else l=mid+1;
    }
    cout<<kq;
}
int main()
{
    ios_base::sync_with_stdio(0);
    cout.tie(0); cin.tie(0);
    freopen("binhphuong.inp","r",stdin);
    freopen("binhphuong.out","w",stdout);
    cin>>k;
    giai();
}