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

int main(int argc,char *argv[]) {
	unsigned int num, i;
	unsigned int long sum = 0;

	if (argc < 2) {
		fprintf (stderr, "Usage: first num\n");
		return 1;
	}

	num = atol (argv[1]);

	for (i = 0; i < num; i++)
		sum += i;

	printf ("%ld\n", sum);

	return 0;
}