input: Write a python function to calculate the sum of comma-separated numbers.
  assert sum_of_numbers('1,2,3,4,5') == 15
  assert sum_of_numbers('10,20,30,40') == 100
  assert sum_of_numbers('7,14,21') == 42
output: def sum_of_numbers(numbers):
      return sum(int(num) for num in numbers.split(','))