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 read_file(source_file, target_file):
  sum = 0
  target = open(target_file, 'w')
  with open(source_file) as f:
    for line in f:
      sum += int(line)
      target.write(line)
  target.write(sum)