def dominator(a: list[int]) -> int:
    size = value = index = 0
    for k, v in enumerate(a):
        if size == 0:
            size += 1
            value = v
            index = k
        elif value != v:
            size -= 1
        else:
            size += 1
    candidate = value if size > 0 else -1

    count = sum(1 for v in a if v == candidate)
    if count <= len(a) / 2:
        index = -1

    return index