input: Write a function to calculate the Fibonacci numbers up to a given number. The function should take an integer `n` as input and return the Fibonacci number at the nth position.
  
  assert Fibonacci(0)==0
  assert Fibonacci(1)==1
  assert Fibonacci(5)==5
  assert Fibonacci(10)==55
  assert Fibonacci(15)==610
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