Given n identical balls and m boxes. Count the number of ways to distribute n balls into m boxes so that there is no box contains more than k balls. Order of boxes is not important, i.e. (1-2-1) and (1-1-2) are the same and must be counted one way.
Constraints: n, m, k <= 1000
Example: n = 4; m = 3; k = 2 -> answer = 2 (2 ways to distribute are (1-1-2) and (0-2-2))
For Vietnamese, you can read the statement here.