input: Write a function to calculate the Fibonacci number at a given position. The function should return the Fibonacci number for the input position n.
  assert fibonacci(10)==55
  assert fibonacci(5)==5
  assert fibonacci(2)==1
  assert fibonacci(0)==0
  assert fibonacci(1)==1
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