# コントラクト付き関数の例（PEP 316 スタイル）
def binary_search(items: list, target: int) -> int:
    """
    pre: items == sorted(items)
    post: __return__ == -1 or items[__return__] == target
    """
    lo, hi = 0, len(items) - 1
    while lo <= hi:
        mid = (lo + hi) // 2
        if items[mid] == target:
            return mid
        elif items[mid] < target:
            lo = mid + 1
        else:
            hi = mid - 1
    return -1
