First of all, make sure there must be 50;
2. From the front (1-25), if you take 1, you won't get 2. Similarly, if you get a 3, you won't get a 6 until you are 25. ...
The first part * * * includes 1, 3,4,5,7,9, 1 1, 12, 13, 15, 16.
A * * * has 17.
So a * * * has 67!
Spent a lot of time, hope to adopt.