#!/bin/ruby

n = gets.strip.to_i
a = gets.strip.split(' ').map(&:to_i).sort!
arr = []
n.downto(2) do |i|
  a.each_cons(i).each do |ec|
    arr.push(ec) if ec.combination(2).all? {|c| c.inject(:-).abs <= 1 }
  end
end
puts arr.map {|e| e.size }.max