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

int main(int argc, char **argv)
{

    int i;
    if (argc == 1) {
	fprintf (stderr, "Usage: first num\n");
	return -1;
    }

    i = atoi(argv[1]);
    printf ("%d\n", i*(i-1)/2);
    return 0;
}