def calc7(org_list):

    ret_list = org_list
    ret_num = -1
    lucnum = tmp = 0
    lucky = False

    if len(org_list) == 0:
        return ret_num

    for i in range(len(org_list)):

        tmp = ret_list.pop()
        ret_num = ret_num  + tmp

    return ret_num

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