input: 
  def max_revenue(prices):
      """Calculate the maximum revenue that can be obtained from a list of prices.
      The list represents the price of an asset over different time points.
      The goal is to find the maximum profit by buying and selling the asset at two different time points.
      >>> 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:     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