input: Write a function to calculate the Fibonacci number at a given position. The function should take an integer \( n \) as input and return the Fibonacci number at that position. If \( n \) is 0, return 0. If \( n \) is 1, return 1. For any other value of \( n \), return the sum of the Fibonacci numbers at positions \( n-1 \) and \( n-2 \).
  assert fibonacci(10)==55
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(5)==5
  assert fibonacci(7)==13
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)