Set Partition (subset)

Problem Background

Special for beginners, ^_^

Problem Description

Let $S$ be a set with $n$ elements, $S = \{a_1, a_2, \dots, a_n\}$. Partition $S$ into $k$ subsets $S_1, S_2, \dots, S_k$ satisfying the following conditions:

1. $S_i \neq \phi$
2. $S_i \cap S_j = \phi\; (1 \le i, j \le k,\; i \neq j)$
3. $S_1 \cup S_2 \cup S_3 \cup \cdots \cup S_k = S$

Then $S_1, S_2, \dots, S_k$ is called a partition of $S$. It is equivalent to placing the $n$ elements $a_1, a_2, \dots, a_n$ of $S$ into $k$ unlabeled boxes $(0 < k \le n < 30)$ such that no box is empty.

Please determine the number of partitions $S(n,k)$ of putting $n$ elements $a_1, a_2, \dots, a_n$ into $k$ unlabeled boxes.

Input Format

Input two integers $n$ and $k$, both less than 30.

Output Format

Output an integer, the number of partitions.

Sample Input

23 7

Sample Output

4382641999117305

Constraints

Time limit: 1s, memory limit: 1024KiB for each test case.