def calc7(num_list):
    if len(num_list) == 0:
        return -1

    sum = 0
    lucky_flg = False

    for i in num_list:
        if lucky_flg:
            sum += 2 * i
        else:
            sum += i
        if i == 7:
            lucky_flg = True
        else:
            lucky_flg = False

    return sum


print(calc7([1, 2]))
print(calc7([3, 7]))
print(calc7([7, 5, 6]))
print(calc7([7, 9, 7, 9, 7, 9]))
print(calc7([]))