#!/usr/bin/perl -w
use strict;

my $sum=0;
foreach my $num (1..1000)
{
   if ($num % 2)       #odd
   {
      $sum += $num
   }
   else                #even
   {
      $sum -= $num;
   }
}

print $sum;
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It is possible to write a closed form expression that generates $sum for any X without using a computing loop. I leave that part of your homework for you to figure out.