input: Write a function to calculate the Fibonacci numbers up to a given number. The function should return the Fibonacci number corresponding to the input number. If the input number is 0, the function should return 0. If the input number is 1, the function should return 1. For any other number, the function should return the sum of the two preceding Fibonacci numbers.
  
  def Fibonacci(n): 
      if n==0: 
          return 0
      if 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(3)==2
  assert Fibonacci(4)==3
  assert Fibonacci(5)==5
  assert Fibonacci(6)==8
output: def Fibonacci(n): 
      if 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