#!/usr/bin/perl

open($f, "progression.in");
open($g, ">progression.out");

$eilute=<$f>;
@eilutes=<$f>;
$eilute=join('',@eilutes);

# print($g "$eilute\n");

$eilute=~s/(-??\d+)/$1\n$1/gm;
$eilute=~s/-??\d+//m;
$eilute.="a";
$eilute=~s/-??\d+\na//m;
$eilute=~s/(-??\d+)\n(-??\d+)/$dif=$1-$2;$dif/gem;

# print($g "$eilute\n");

$max=0;
$eilute=~s/(-??\d+)/$1\n$1/gm;
$eilute=~s/-??\d+//m;
$eilute.="a";
$eilute=~s/-??\d+\na//m;
# $eilute=reverse $eilute;
# $eilute=~s/-??\d+//;
# $eilute=reverse $eilute;
$eilute=~s/(-??\d+)\n(-??\d+)/$1==$2 ? $max++ : {$max=0} ;$max/gem;

# print($g "$eilute");

$max=0;
$eilute=~s/(\d+)/ $max=$1 if ($max<$1) ;$1/gem;

print($g $max+2);