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