Didn't understand the output of the following Python 3 code with reduce function?

[David Raymond]

All the numbers in the nums list don't matter and aren't used. Only the first number, and how many there are.
https://docs.python.org/3.8/library/functools.html#functools.reduce

Basically it's doing
ADDS(1, 2) which returns 2
that 2 gets fed back into
ADDS(2, 3) which returns 3
that 3 gets fed back into
ADDS(3, 4) which returns 4
...
ADDS(8, 9) which returns 9

> I have seen this code on one of competative programming site but I didn't get it, Why output is 9?
> 
> from functools import *
> 
> def ADDS(a,b):
>     return a+1
> nums = [1, 2, 3, 4, 5, 6, 7, 8, 9]
> add = reduce(ADDS, nums)
> print(add)
> 
> output: 9