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

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

puts hakake(numbers)