#!/usr/bin/perl -l

# Libraries
use strict;
use warnings;
use Getopt::Std;
use File::Basename;

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

# Globals
my $iam = basename $0;
my $syntax = "
    syntax:  $iam  <first number>  <second number>
 
    Your syntax message here.

";

# Command-line
(my $x = shift) or die $syntax;
(my $y = shift) or die $syntax;

# Validity checking
($x =~ /^-?\d+$/) or die "$iam:  value $x not an integer\n";
($y =~ /^-?\d+$/) or die "$iam:  value $y not an integer\n";

# Main program
if ($opt_s) {
    printf "%d\n", ($y - $x + 1) * ($x + $y) / 2;
} else {
    printf "%d\n", ($y - $x) * ($x + $y + 1) / 2;
}