#! perl -slw
use strict;

our $h;
our $s;
die <<usage if $h or @ARGV != 2;
    $0 [-s] low high

    Calculates sum( low+1 .. high )
    With -s calculates sum( low .. high )
    (integers only)
usage

my( $low, $high ) = @ARGV;

die "Arguments must be integers\n"
    unless $low =~ m[^\d+$] and $high =~ m[^\d+$];

my $result = ( $low + $high ) * ( $high - $low + 1 ) / 2;
$result -= $low unless $s;

print $result;