#include <stdlib.h>
#include <stdio.h>

int main()
{
    int n;
    freopen("fact.in", "rt", stdin);
    freopen("fact.out", "wt", stdout);
    scanf("%d", &n);
    for(int i=1;i<=n;i++)
    {
        n=n+i;


    }
    printf("%d", n);
    return 0;
}