```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, bits, length):
    if not _whole(bits) or bits <= 0:
        raise ValueError("capacity must be a positive integer")
    if not _whole(length) or length <= 0:
        raise ValueError("run length must be a positive integer")
    if bits * WORD_BITS < length:
        raise ValueError("run length exceeds the capacity")
    for i in range(bits * WORD_BITS):
        if _bit_of(words, i) == 1:
            raise ValueError("bitmap word is occupied")
    start = 0
    while start < bits * WORD_BITS:
        if _bit_of(words, start) == 0:
            break
        start += 1
    if start == bits * WORD_BITS:
        return -1
    for i in range(start, start + length):
        if _bit_of(words, i) == 1:
            return -1
    return start
```