Loop sum of the list until length=1. I really need some help to optimize

num_lst = list(map(int,input()))
b = sum(num_lst)
if len(str(b))==1:
    print(b)
else:
    while True:
        c = str(b)
        new_list = list(map(int,c))
        d = sum(new_list)
        if len(str(d))==1:
            break
    print(d)

These are the code I’ve been trying but the problem is when I run it with too many length of input. It’s timeout. (I am really new to coding as you can see).
Here is the example.
Input: 12345
Output: 6 from 1+2+3+4+5 = 15 and then 1+5 =6.
But when I input like "67896789678967896879678967896789678967896789678969".
It’s timeout.

>Solution :

To solve this problem just change the variable d in the loop by the variable b. As you do with each iteration of the while it will pick up the variable b again, which you didn’t change the value of, thus creating an infinite loop

num_lst = list(map(int,input()))
b = sum(num_lst)
if len(str(b))==1:
    print(b)
else:
    while True:
        c = str(b)
        new_list = list(map(int,c))
        b = sum(new_list)
        if len(str(b))==1:
            break
    print(b)