input: Write a function to calculate the Fibonacci number at a given position. The function should return the Fibonacci number for the position n. If n is 0, return 0; if n is 1, return 1; otherwise, return the sum of the two preceding Fibonacci numbers.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(20) == 6765
  assert fibonacci(5) == 5
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