input: Write a python function to determine if a number is an abundant number.
  assert is_abundant(12) == True
  assert is_abundant(13) == False
  assert is_abundant(9) == False
output: def is_abundant(n):
      if n < 1:
          return False
      divisors_sum = 0
      for i in range(1, n):
          if n % i == 0:
              divisors_sum += i
      return divisors_sum > n