10079 Pizza Cutting by loop

#include <stdio.h>

int main()
{
    long long int n, sum = 0, i;
    scanf("%lld", &n);
    while (n >= 0)
    {
        sum = 1;
        for (i = 0; i <= n; i++)
            sum += i;

        printf("%lld\n", sum);
        scanf("%lld", &n);
    }
    return 0;
}