void destroy (Number *head)
{
    Number *pom = head;
    if (pom == NULL)
    {
        printf ("Lista pusta.");
    }
    if (pom->next == NULL)
    {
        free(pom);
        head = NULL;
    }
    else
    {
        while ((pom->next)->next != NULL)
        {
            pom = pom->next;
        }
        free (pom->next);
        pom->next = NULL;
    }
}