#!/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 integers as
arguments and produces the sum of the range of integers
starting after the first argument and ending with the
second argument.
';

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

HELP_MESSAGE() and exit unless $ARGV[1];

foreach (@ARGV) {
  HELP_MESSAGE() and exit unless /^-?\d+$/
}

my $start = shift @ARGV;
my $end = shift @ARGV;

if ($opt_s) {
  print( ($end - $start + 1) * ($start + $end) / 2 )
} else {
  print( ($end - $start) * ($start + $end + 1) / 2 )
}

sub HELP_MESSAGE {
  print $help;
}