#!/usr/bin/perl -w

use strict;
use warnings;

my $ABOUT = "This program adds two positive integers of any size and displays the result.\n" .
            "Usage: ADD <number> <number>\n\n" .
            "Example: ADD 19333045834565223108349833745893453 12982372388169001\n\n" .
            "This program requires two numbers as parameters.\n" .
            "The parameters may only contain digits (0-9).\n\n";

@ARGV == 2 or die $ABOUT;

if (Is_Not_A_Number($ARGV[0]) || Is_Not_A_Number($ARGV[1]))
{ die $ABOUT; }

print ADD($ARGV[0], $ARGV[1]);
exit;

####################################

sub ADD
{
  my $A = defined $_[0] ? $_[0] : '';
  my $B = defined $_[1] ? $_[1] : '';
  my $AL = length($A);
  my $BL = length($B);
  my $i = ($AL > $BL) ? $AL : $BL;
  my $CARRY = 0;
  my $SUM = '0';
  my $X;
  while ($i-- > 0)
  {
    $X  =  $AL ? vec($A, --$AL, 8) : 48;
    $X += ($BL ? vec($B, --$BL, 8) : 48) + $CARRY;
    $CARRY = $X > 105 ? 1 : 0;
    vec($SUM, $i, 8) = $X - ($CARRY ? 58 : 48);
  }
  return ($CARRY ? '1' : '') . $SUM;
}

####################################

sub Is_Not_A_Number
{
  @_ or return 1;
  my $N = shift;
  return 1 unless defined $N;
  my $L = length($N);
  return 1 unless $L;
  my $C;
  while ($L-- > 0)
  {
    $C = vec($N, $L, 8);
    return 1 if ($C < 48 || $C > 57);
  }
  return 0;
}