input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number for the given input n. If n is 0, the function should return 0. If n is 1, the function should return 1. For any other value of n, the function should return the sum of the two preceding Fibonacci numbers.
  assert fibonacci(6)==8
  assert fibonacci(8)==21
  assert fibonacci(10)==55
  assert fibonacci(0)==0
  assert fibonacci(1)==1
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