#include <stdio.h>
int main ()
{
    int m ,i, sum;
    scanf("%d", &m);
    if(m >= 0){
         sum = (m*(m+1))/2 ;
         printf("%d\n", sum);
         }
    else{
        for(i=1, sum =0; i>=m; i-- ){
            sum = sum+i;
        }
        printf("%d\n", sum);
    }
    return 0;
}