use strict;
use warnings;

my $str;

while (1) {
    print "Please enter the number.(Enter exit at the end) ";
    $str = <STDIN>;
    chomp $str;
    
    if ($str eq "exit" || $str eq "") {
        last;
    }
    
    if ($str =~ /[a-zA-Z]/) {
        print "Not number.\n";
        next;
    }
    
    print (abs($str), "\n");
};