input: Write a function to generate the nth number in the Fibonacci sequence. If the input n is less than 0, return "Incorrect input". If n is 0, return 0. If n is 1, return 1. Otherwise, return the nth Fibonacci number.
  def fibonacci(n): 
      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 
  
  assert fibonacci(9)==34
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(-1)=="Incorrect input"
  assert fibonacci(5)==5
output: def fibonacci(n): 
      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