符合要求的代码怎样编写？

输入样本可以得到正确结果

code = input("输入12位数字")
a = 0
for i in range(len(code)):
    if i%2 == 0:
        a += int(code[i])
    else:
        a += 3 * int(code[i])   
last_num = 10 - a % 10
code = code + str(last_num if last_num != 10 else 0)