```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) != capacity // WORD_BITS + (capacity % WORD_BITS > 0):
        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 total number of bits available
    total_bits = len(words) * WORD_BITS
    
    # Iterate over possible start positions for the free run
    for start in range(total_bits - length + 1):
        # Check if the run lies entirely below the capacity
        if start + length <= total_bits:
            # Check if there are no occupied blocks in the run
            if all(_bit_of(words, start + i) == 0 for i in range(length)):
                return start
    
    return -1
```