def hoge
  n = gets.to_i
  h = gets.split.map(&:to_i)
  h << Float::INFINITY
  flag = false
  n.times do |i|
    if h[i+1] < h[i]
      if flag
        puts 'No'
        return
      else
        flag = true
      end
    else
      flag = false
    end
  end
  puts 'Yes'
end

hoge