input: Write a function to calculate the nth Fibonacci number. The Fibonacci sequence is defined as: F(0) = 0, F(1) = 1, and for n > 1, F(n) = F(n-1) + F(n-2). If n is negative, return "Incorrect input". If n is 0, return 0. If n is 1, return 1.
  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(1)==1
  assert fibonacci(0)==0
  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