input: Write a function to calculate the Fibonacci sequence. The function should take an integer n as input and return the nth number in the Fibonacci sequence. If the input n is less than 0, the function should print "Incorrect input" and return None.
  def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
          return None
      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 
  
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(9)==34
  assert fibonacci(-1) == None
output: def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
          return None
      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