n = gets.to_i
pos = gets.split(' ').map(&:to_i).sort!
diff = pos[1] - pos[0]
if diff == 0
	puts "NO"
	exit
end
(1...n-1).each do |i|
	if pos[i + 1] - pos[i] != diff
		puts "NO"
		exit
	end
end
puts "YES"