#!/usr/bin/perl -l

use strict;
use warnings;

use Getopt::Std;

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

# get the range boundaries
my $range_start  = ( $opt_s ) ? $ARGV[0] : $ARGV[0] + 1;
my $range_end    = $ARGV[1];

# compute the sum of the range
my $range_length = $range_end - $range_start + 1;
my $range_sum    = ($range_start + $range_end) * $range_length / 2;

print $range_sum;

##</code><code>##

my $range_start = $ARGV[0];
$range_start++ if ( $opt_s );

##</code><code>##

if (    $ARGV[0] !~ m{ \A [+-]? \d+ \z }xms
     || $ARGV[1] !~ m{ \A [+-]? \d+ \z }xms ) {
    die "usage: $0 [-s] <integer> <integer>\n";
}