outputting strings and function outputs in Python

I am coding a simple program to add all positive integers not greater than a given integer n. My code:

print("Enter an integer:")
n=input()

def add(k):
    sum=0
    for i in range(k+1):
        sum=sum+i
    return sum
    
#print("1+2+3+...+"+str(n)+"="+str(add(n)))

print(add(100))

The function works.

Why does the line in the one line comment not work, if I remove the hash tag? It should – there is a concatenation of four strings. Thank you.

EDIT: the whole output:

Enter an integer:
12
Traceback (most recent call last):
File "<string>", line 10, in <module>
  File "<string>", line 6, in add
TypeError: can only concatenate str (not "int") to str
>

>Solution :

input() returns a string. You are passing n=input() which is a string so it is not working as expected.
change it to n=int(input())

Also sum is a reserved keyword and it will be best to change that to a different name