input: Write a python function to calculate the sum of the first N natural numbers using a while loop.
  assert sum_of_natural_numbers(10) == 55
  assert sum_of_natural_numbers(5) == 15
  assert sum_of_natural_numbers(0) == 0
output: def sum_of_natural_numbers(N):
      sum = 0
      i = 1
      while i <= N:
          sum += i
          i += 1
      return sum