input: Write a function to calculate the nth Fibonacci number. Return the nth Fibonacci number. If the input is negative, print "Incorrect input" and return None. If the input is 0, return 0. If the input is 1, return 1. For any other positive integer, return the nth Fibonacci number using an iterative approach.
  
  assert fibonacci(9)==34
  assert fibonacci(1)==1
  assert fibonacci(0)==0
  assert fibonacci(-1)==None
  assert fibonacci(5)==5
output: def fibonacci(n):
      if n < 0:
          print("Incorrect input")
          return None
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b