#!/usr/bin/python3

from functools import lru_cache

adapters = []
with open("input") as fh:
    for ln in fh:
        adapters.append(int(ln))

adapters = sorted(adapters)

@lru_cache(maxsize=None)
def count(start, offset):
    if offset == len(adapters) - 1:
        return 1
    n = 0
    for i in range(offset, len(adapters)):
        if adapters[i] - start < 1:
            raise RuntimeError()
        elif adapters[i] - start <= 3:
            n += count(adapters[i], i+1)
        else:
            break
    return n

print(count(0, 0))