#!/usr/bin/perl
use strict;
use warnings;
use Getopt::Std;

getopts( 's', \my %opt );

print(
   ( $ARGV[ 1 ] - $ARGV[ 0 ] + $opt{ s } ? 1 : 0 ) *
   ( $ARGV[ 0 ] + $ARGV[ 1 ] + $opt{ s } ? 0 : 1 ) / 2,
   "\n"
     );