func_code: def fibonacci(n):
      if n == 0:
          return '0'
      elif n == 1:
          return '1'
      else:
          n1 = 1
          n2 = 0
          i = 2
          while i <= n:
              sum1 = n1 + n2
              n2 = n1
              n1 = sum1
              i = i + 1
          return sum1