gets

nums = gets.split(" ").map(&:to_i)
ans = "Yes"
(0..(nums.length - 2)).each do |i|
  if nums[i] < nums[i + 1]
    nums[i + 1] -= 1
  elsif nums[i] > nums[i + 1]
    ans = "No"
    break
  end
end

puts ans
