import sys, json

class Solution:
    def longestConsecutive(self, nums: list[int]) -> int:
        if len(nums) == 0:
            return 0

        nums.sort()
        results = [0 for _ in range(len(nums))]

        for i in range(1, len(nums)):
            if i == 0:
                results[i] = 1
            elif nums[i] - nums[i - 1] == 1:
                results[i] = max(results[i - 1] + 1, results[i - 2])
            else:
                results[i] = results[i - 2]

        print(results)
        return results[-1]

data = json.loads(sys.stdin.read())
result = Solution().longestConsecutive(data["nums"])
print(result)