with open("input") as f:
    data = f.read()

blocks = []
last_file_id = None
for idx, count in enumerate(data):
    block_char = "." if idx % 2 == 1 else str(idx // 2)
    for ii in range(int(count)):
        blocks.append(block_char)
        last_file_id = block_char

end_seek_idx = len(blocks) - 1
for file in range(int(last_file_id), -1, -1):
    file_block = str(file)
    file_start_block = None
    file_end_block = None
    for idx in range(end_seek_idx, -1, -1):
        if blocks[idx] == file_block:
            if file_end_block is None:
                file_end_block = idx
        else:
            if file_end_block is not None:
                file_start_block = idx + 1
                end_seek_idx = idx
                break
    else:
        break

    file_size = file_end_block - file_start_block + 1

    empty_block_size = 0
    for idx, block_content in enumerate(blocks):
        if idx > end_seek_idx:
            break

        if block_content == ".":
            empty_block_size += 1
        else:
            empty_block_size = 0

        if empty_block_size == file_size:
            empty_block_start = idx - file_size + 1
            for ii in range(file_size):
                blocks[empty_block_start + ii], blocks[file_start_block + ii] = blocks[file_start_block + ii], blocks[empty_block_start + ii]
            break

def calculate_checksum(blocks: list[str]):
    checksum = 0
    for index, elem in enumerate(blocks):
        if elem == ".":
            continue

        checksum += index * int(elem)

    return checksum


print(calculate_checksum(blocks))