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

int main(void){
    char a[4];
    int b;
    int sum;
    
    fgets(a, sizeof(a), stdin);
    b = atoi(a);
    
    sum = b;
    while(b != 0){
        b--;
        sum += b;
    }
    
    printf("%d", sum);
    
    return 0;
}