input: 
  def max_revenue(prices):
      """Calculate the maximum revenue obtainable by buying and selling the stock on adjacent days.
      >>> max_revenue([10, 7, 5, 8, 11, 9])
      6
      >>> max_revenue([1, 2, 3, 4, 5])
      4
      """
      diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      for i in range(len(diff)): 
          for j in range(i + 1, len(diff) + 1): 
              current_diff = sum(diff[i:j]) 
              if current_diff > max_diff: 
                  max_diff = current_diff 
   
      return max_diff
  
output:     diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      current_sum = 0
      for d in diff:
          current_sum = max(0, current_sum + d)
          max_diff = max(max_diff, current_sum)
      return max_diff