#include <iostream>
using namespace std;
int main()
{
    int a=1,b=1,c=0,n;
    cin>>n;
    while(a<=n)
    {
        for(int i=1;i<=b;i++)
        {
            c=c+b;
            a++;
            if(a>b)
                break;
        }
        b++;
    }
    cout<<c;
    return 0;
}