using namespace std;
int main()
{
    int n;
    cin>>n;
    int sum = 0, d = 1, j = 1;
    for(int i = 1; i <= n; ++i)
    {
        sum += d;
        if(j >= d) 
        {
            d++;
            j = 1;
        }
        else
            j++;
    }
    cout<<sum;
    return 0;
}