data = []

with open("input.txt") as file:
  for line in file:
    _line = line.rstrip()
    data.append(int(_line))


def solve_p1(input_data) -> int:
  _sorted_data = sorted(data)
  sorted_data = [0] + _sorted_data + [_sorted_data[-1] + 3]
  diff = {
    1: 0,
    2: 0,
    3: 0
  }
  for i in range(1, len(sorted_data)):
    diff[sorted_data[i] - sorted_data[i-1]] += 1
  return diff[1] * diff[3]

# Warning
# this is correct algorithmically, but naive, so extremely slow for long lists
# Saving this because it's nevertheless interesting approach IMO
def naive_solve_p2(input_data) -> int:
  _sorted_data = sorted(input_data)
  sorted_data = [0] + _sorted_data + [_sorted_data[-1] + 3]
  
  def rec_seek_remove(start_index, partial_data):
    result = 1  # we start with 1 because we have the option to do nothing
  
    for i in range(start_index, len(partial_data)-1):
      diff = partial_data[i+1] - partial_data[i-1]
      if diff <= 3: # can be deleted
        new_list = partial_data[:i] + partial_data[i+1:]
        result += rec_seek_remove(i, new_list)

    return result

  return rec_seek_remove(1, sorted_data)


print(solve_p1(data))