adapters = []
with open("input.txt") as f:
	adapters = list(int(num) for num in f.read().split("\n"))

adapters.append(0)
adapters.sort()
adapters.append(adapters[-1]+3)

memo = {}
memo[adapters[-1]] = 1
def getNumOfCombinations(adapterList):
	if memo.get(adapterList[0]):
		 return memo[adapterList[0]]
	combinations = 1
	for i in range(1, len(adapterList)):
		diff = adapterList[i] - adapterList[0]
		if 0 < diff <= 3:
			 combinations += getNumOfCombinations(adapterList[i:])
		else:
			break
	combinations = combinations - 1
	memo[adapterList[0]] = combinations
	return combinations

print(getNumOfCombinations(adapters))