def search(bs, arr):
    """
    Binary search recursive function.
    :param bs: int
        What to search
    :param arr: array
        Array of ints where to search. Array must be sorted.
    :return: string
        Position of required element or "cannot found" string
    """

    length = len(arr)
    check_item = arr[int(length / 2)]

    if check_item == bs:
        return str(check_item) + ' is on position ' + str(arr[int(length / 2)])

    if check_item < bs:
        return search(bs, arr[int(length / 2):])

    if check_item > bs:
        return search(bs, arr[:int(length / 2)])

    return "cannot found"