#include <stdio.h>

int main(void) {
    int n;
    int sum = 0;

numInput:
    printf("Input integer: ");
    scanf("%d", &n);

    if (n < 1) {
        puts("Error: Please type in non-zero positive integer.");
        goto numInput;
    }

    for (int i = 1; i <= n; i++) {
        sum += i;
    }

    printf("ANS: %d", sum);

    return 0;
}