#include <stdio.h>
int main()
{
	long int n = 0;
	long int ret = 0;
	while (scanf("%ld", &n) != EOF)
	{
		if (n == 0)
			return 0;
		while (n)
		{
			ret = ret + n;
			n = n - 1;
		}
		printf("%ld\n", ret);
	}
	return 0;
}