Adding 2-column CSV together in base Python without using pandas

[LeegeeRicky]

I have a CSV file with ID column (Username) and two numeric columns. In base Python I want to get the ID and the sum of Auto and Manual Score, then generate another CSV with the result.

Example input CSV:
Username Auto Score Manual Score
1234, 1,
1234, 1,
1234, 1,
1234, , 1.5
345, 1,
345, 1,
345, , 2
133, 1,
133, 1,
133, , 2.5

Here is my code:

import csv

with open("exam_for_2019.csv") as csvfile:
    reader = csv.DictReader(csvfile)
    
    for row in reader:
        sum_row = int(row['Auto Score']) + int(row['Manual Score'])
        print(sum_row)

But I got the error as: ValueError: invalid literal for int() with base 10: '' .

How can I fix it to sum the result with one Username with one sum score each row?