Print a number not in a list python

I’m having an issue with a simple task it seems, but cannot figure it out. I believe the solution is within the code:

n = input().split(',')

list1 = []
list2 = []
for x in n:
    list1.append(int(x))

for y in range(1, len(list1 + 1)):
    if y not in list1:
        list2.append(y)
print(list2)

The task is:
Given an array of integers, some elements appear twice and others appear once.
Each integer is in the range of [1, N], where N is the number of elements in the array.

Find all the integers of [1, N] inclusive that do NOT appear in this array.

Constrain:
N will always be in the range of [5, 1000]

Input:
1,2,3,3,5
Output:
4
Input:
1,1,1,1,1,1,1,1
Output:
2,3,4,5,6,7,8

My idea is to have two empty arrays. The first one I will write all the numbers using a for loop. Once I have them all there I will use another loop where I can look and find the missing numbers. I guess it related to some formatting that is killing my logic.
Any help would be appreciated!
Thanks

>Solution :

You only have to change this line:

for y in range(1, len(list1 + 1)):

to this one:

for y in range(1, len(list1)+1):

The problem is that you added one to the list, but you want to add 1 to the length of the list.