perl -e 'for$c(sort{$a<=>$b}map/(\d+)$/,<*[0-9]>){++$n==$c||print map"$_\n",$n..$c-1;$n=$c}'

##</code><code>##

perl -le '@f=map/(\d+)$/,<*[0-9]>;@x[@f]=@f;for(1..$#x){$x[$_]||print}'