read from source file and write to target file line by line until the end of the source file. use a function to add the numbers in the source file and write to target file the sum.

def add(a, b):
    return a + b

def read(source, target):
    with open(source, 'r') as s, open(target, 'w') as t:
        line1 = s.readline()
        line2 = s.readline()
        t.write(add(int(line1), int(line2)))
    return
    
read('number1.txt', 'number2.txt')