print map { @$_, "\n" } split_into_three( 'a'..($ARGV[0] || 'z') );

sub split_into_three {
        my @a = @_;
        my (@b, @c);
        while (@b < @a) {
                push(@b, shift(@a));
                unshift(@c, pop(@a));
        }
        push(@a, shift(@c)) if (@a < @c);
        return \@b, \@a, \@c;
}