input: 
  from typing import List
  
  def max_revenue(prices: List[int]) -> int:
      """ Calculate the maximum revenue that can be obtained by buying and selling a stock.
      The input is a list of stock prices where consecutive prices represent the price of the stock 
      on consecutive days. The goal is to find the maximum revenue possible by buying on one day 
      and selling on another day.
      >>> max_revenue([10, 7, 5, 8, 11, 9])
      6
      """
      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:     if not prices:
          return 0
      
      min_price = prices[0]
      max_profit = 0
      
      for price in prices:
          if price < min_price:
              min_price = price
          else:
              max_profit = max(max_profit, price - min_price)
      
      return max_profit