from collections import defaultdict

with open('input', 'r') as f:
    adapters = list(map(int, f.readlines()))

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

countMap = defaultdict(int)
countMap[0] = 1

for adapter in adapters:
    for target in range(1, 4):
        countMap[adapter + target] += countMap[adapter]

print(countMap[adapters[-1]])