input: Write a function to calculate the Fibonacci number at a given position. If the input position is 0, return 0. If the input position is 1, return 1. For any other position, return the sum of the Fibonacci numbers at the previous two positions.
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(10)==55
  assert fibonacci(5)==5
  assert fibonacci(7)==13
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