ifile = "input.txt"

    adapters = [int(line.strip()) for line in open(ifile).readlines()]

    def possibilities(jolts, adapter_idx):
        ans = 0
        if adapter_idx == len(adapters):
            return 1
        for idx in range(adapter_idx, len(adapters)):
            if(jolts <= adapters[idx] <= jolts + 3):
                ans += possibilities(adapters[idx], idx+1)
            elif(adapters[idx]> jolts + 3):
                break
        return ans

    adapters.sort()
    print(possibilities(0, 0))