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. If the input is 0, return 0. If the input is 1, return 1. For any other input, return the sum of the two preceding Fibonacci numbers.
  assert Fibonacci(0)==0
  assert Fibonacci(1)==1
  assert Fibonacci(5)==5
  assert Fibonacci(10)==55
  assert Fibonacci(20)==6765
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