input: def fibonacci(n: int) -> int:
      a = 0
      b = 1
      if n < 0: 
          return "Incorrect input"
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2, n+1): 
              c = a + b 
              a = b 
              b = c 
          return b 
  
  # Test cases
  # print(fibonacci(0)) # Output: 0
  # print(fibonacci(1)) # Output: 1
  # print(fibonacci(9)) # Output: 34
output:     a = 0
      b = 1
      if n < 0: 
          return "Incorrect input"
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2, n+1): 
              c = a + b 
              a = b 
              b = c 
          return b