#!/usr/bin/perl -l
use strict;
use warnings;

use Getopt::Std;

our $VERSION = '1.0';

$Getopt::Std::STANDARD_HELP_VERSION = 'true';

my $help = '
syntax:
  series [-s] <first integer> <second integer>

--help
    prints this help text and exits: invoking the help
    argument causes all other arguments to be discarded by
    this utility

-s
    takes no arguments: changes program behavior so that the
    first number specified is the first number of the range,
    and not the number preceding it (syntax: series a+1 n)
    -- this option must precede all numerical arguments to
    the series utility

description:  This utility takes two positive integers as
arguments and produces the sum of the range of integers
starting after the first argument and ending with the second
argument.  It allows 0 to be specified as the first
argument, indicating that all numbers in the range are
positive integers.

credits:  Chad L. Perrin (author) 
          PerlMonks community (contributors) -- node 594743

license:  This software is licensed CCD CopyWrite.  See the 
webpage at http://ccd.apotheon.org for licensing details.
';    

our ($opt_s);
getopts('s');

die $help unless defined $ARGV[1];
map { /^-?\d+$/ or die "Invalid input: '$_'$!" } @ARGV;
die "Error: arguments in descending value order.\n$help" if ($ARGV[0] > $ARGV[1]);

my $start = $ARGV[0] + ( $opt_s ? 0 : 1);
my $end = $ARGV[1];

print( ($end - $start + 1) * ($start + $end) / 2 );

sub HELP_MESSAGE { 
  print $help 
}