def get_student_avg(gradebook_dict, student):
    """ Given a dictionary where each key-value pair is of the form: (student_name, [scores]),
    return the average score of the given student. If the given student does not exist, return -1

    Example:
    >>> get_student_avg({"Sally":[80, 90, 100], "Harry": [75, 80, 85]}, "Sally")
    90.0
    >>> get_student_avg({"Sally":[80, 90, 100], "Harry": [75, 80, 85]}, "John")
    -1
    """
    if student not in gradebook_dict: 
        return -1
    else: 
        sum = 0
        count = 0
        for i in gradebook_dict[student]: 
            sum += i
            count += 1
        return sum / count