input: Write a function to calculate the Fibonacci number at a given position. The function should return the Fibonacci number at the specified position. If the given position is 0, the function should return 0. If the given position is 1, the function should return 1. For any other position, the function should return the sum of the two preceding Fibonacci numbers.
  assert fibonacci(10)==55
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(7)==13
  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