n = gets.to_i
x_list = gets.split.map(&:to_i)

x_list.sort!

f_diff = x_list[1] - x_list[0]

ok = x_list[1..-2].zip(x_list[2..-1]).all? do |l, h|
  h - l == f_diff
end

ok = false if f_diff == 0

puts ok ? 'YES' : 'NO'