use strict;
use warnings;
use constant MAX => 1e6;

$| = 1;

my %seq = (0 => 0);
my $r0  =  0;

for my $n (1 .. MAX)
{
    my $r = $r0 - $n;
       $r = $r0 + $n if $r < 0 || exists $seq{$r};

    $seq{$r} = $n;
    $r0      = $r;
}

for (0 .. MAX)
{
    unless (exists $seq{$_})
    {
        printf "For MAX = %.1e, the first missing number is %d\n", MAX, $_;
        last;
    }
}