input: Write a function to find the nth Fibonacci number. If n is less than 0 return "Incorrect input", if n is 0 return 0, if n is 1 return 1, otherwise return the sum of the previous two Fibonacci numbers.
  def Fibonacci(n): 
      if n<0: 
          print("Incorrect input") 
      elif n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          return Fibonacci(n-1)+Fibonacci(n-2)
  assert Fibonacci(0)==0
  assert Fibonacci(1)==1
  assert Fibonacci(2)==1
  assert Fibonacci(5)==5
  assert Fibonacci(10)==55
output: def Fibonacci(n): 
      if n < 0: 
          return "Incorrect input" 
      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