include <iostream>

include <bits/stdc++.h>

using namespace std;
long long n,z,l[10000],t[10000],i=1;
//int n;
int main()
{
freopen("tpn.inp","r",stdin);
freopen("tpn.out","w",stdout);
cin>>n;
//cout<<n;
while(z<=n)
{
l[i]=iii;
z=l[i];
t[i]=t[i-1]+l[i];
i++;
}
cout<<t[i-2]+l[i-2]*(n-l[i-2]);

return 0;

}