input: Write a function to calculate the nth term of the Fibonacci sequence. The function should return the nth Fibonacci number. If the input is negative, the function should print "Incorrect input".
  def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2,n): 
              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) is None
output: def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("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