Given the task definition and input, reply with output. The task is to generate the correct answer for a given math problem.

Problem: there are 6 boxes numbered 1 , 2 , , . 6 . each box is to be filled up either with a red or a green ball in such a way that at least 1 box contains a green ball and the boxes containing green balls are consecutively numbered . the total number of ways in which this can be done is
21