input: Write a python function to compute the sum of natural numbers up to a given number.
  assert natural_sum(7) == 28
  assert natural_sum(10) == 55
  assert natural_sum(0) == 0
  assert natural_sum(-5) == 0
output: def natural_sum(n):
      if n < 1:
          return 0
      return n * (n + 1) // 2