input: Write a function to calculate the Fibonacci number at a given position. If the position is 0, return 0. If the position is 1, return 1. For any other position, return the sum of the Fibonacci numbers at the two preceding positions.
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  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