numbers=gets.chomp.split.map {|x| x.to_i}

def hakake(nums)
  return 1 if nums[0] != 1
  ret = 10
  for x in 0..(nums.length-1)
    if x > 0 and nums[x] != nums[x-1] + 1
      ret = nums[x] - 1
      break
    end
  end
  return ret
end

puts hakake(numbers)