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

[Shivlal Sharma]

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
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