```python
"""A block-allocation bitmap: 32-bit words, LSB-first, a set bit is occupied."""

WORD_BITS = 32
WORD_MAX = 0xFFFFFFFF


def _whole(value):
    return not isinstance(value, bool) and isinstance(value, int)


def _check_words(words):
    for word in words:
        if not _whole(word) or word < 0 or word > WORD_MAX:
            raise ValueError("bitmap words must be integers from 0 to 4294967295")


def _bit_of(words, index):
    return (words[index // WORD_BITS] >> (index % WORD_BITS)) & 1


def bit_at(words, index):
    _check_words(words)
    if not _whole(index):
        raise ValueError("bit index must be an integer")
    if index < 0 or index >= len(words) * WORD_BITS:
        raise ValueError("bit index is outside the words")
    return _bit_of(words, index)


def occupy_run(words, start, length):
    _check_words(words)
    if not _whole(start) or start < 0:
        raise ValueError("run start must be a non-negative integer")
    if not _whole(length) or length <= 0:
        raise ValueError("run length must be a positive integer")
    if start + length > len(words) * WORD_BITS:
        raise ValueError("run leaves the words")
    for i in range(start, start + length):
        if _bit_of(words, i) == 1:
            raise ValueError("run touches an occupied block")
    marked = list(words)
    for i in range(start, start + length):
        marked[i // WORD_BITS] |= 1 << (i % WORD_BITS)
    return marked


def find_free_run(words: list, capacity: int, length: int) -> int:
    _check_words(words)
    if not _whole(capacity) or capacity <= 0:
        raise ValueError("capacity must be a positive integer")
    if len(words) * WORD_BITS != capacity:
        raise ValueError("word count disagrees with the capacity")
    if not _whole(length) or length <= 0:
        raise ValueError("run length must be a positive integer")

    # Calculate the number of words needed for the given capacity
    num_words = (capacity + WORD_BITS - 1) // WORD_BITS

    # Check if the words list matches the calculated word count
    if len(words) != num_words:
        raise ValueError("word count does not agree with the capacity")

    # Initialize variables to track the free run
    start_of_free_run = None

    # Iterate over each word in the bitmap
    for i in range(num_words):
        word = words[i]

        # Check if the word is within the valid range
        if word < 0 or word > WORD_MAX:
            raise ValueError("bitmap word must be integers from 0 to 4294967295")

        # Count the number of consecutive free bits in this word
        free_bits = 0
        for j in range(WORD_BITS):
            if (word >> j) & 1 == 0:
                free_bits += 1
            else:
                break

        # Check if the current run is complete and update the start_of_free_run
        if free_bits >= length:
            if start_of_free_run is None or i * WORD_BITS < start_of_free_run:
                start_of_free_run = i * WORD_BITS

    # Return the start of the smallest free run, or -1 if none exists
    return start_of_free_run if start_of_free_run is not None else -1
```