Hi everyone. I'm having trouble thinking about a dp problem. The problem is:t There are n students with height from 1->n and one number k(n<=2000, k<=n) You have to line up ALL students so that one person in front of the first person and can see exactly k faces. Example: 2 5 1 6 3 4 => that person can see the face of 1st, 2nd and 4th student. Your work is count how many ways to line up ALL n students. (module 1e9+7)

Test Inp: 3 2 => Output 3 (2 1 3, 1 3 2, 2 3 1)

Thanks for your help.

//Ps Sorry, I am not good at English :((