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#698491. Placing Rooks

时间限制:2 s       空间限制:512 MiB       标签: 数学 斯特林数 第二类斯特林数 CodeForces 缺题解 

算法难度等级:0       思维难度等级:0       实现难度等级:0


本题来源于:Educational Codeforces Round 86 (Rated for Div. 2) Problem E

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  • English

题目描述

n×nn \times n 的国际象棋棋盘上放 nn 个车,要求满足两个条件:

  • 所有的空格子都能被至少一个车攻击到。
  • 恰好kk 对车可以互相攻击到。

答案对 998244353998244353 取模。

输入格式

输入一行两个整数 n,kn,k

输出格式

输出一行一个整数,表示答案。

数据范围

1n2×1051\le n \le 2\times 10^50kn(n1)20 \le k \le \frac{n(n-1)}{2}

样例输入输出

3 2
6
3 3
0
4 0
24
1337 42
807905441

Calculate the number of ways to place nn rooks on n×nn \times n chessboard so that both following conditions are met:

  • each empty cell is under attack;
  • exactly kk pairs of rooks attack each other.

An empty cell is under attack if there is at least one rook in the same row or at least one rook in the same column. Two rooks attack each other if they share the same row or column, and there are no other rooks between them. For example, there are only two pairs of rooks that attack each other in the following picture:

One of the ways to place the rooks for n=3n = 3 and k=2k = 2 Two ways to place the rooks are considered different if there exists at least one cell which is empty in one of the ways but contains a rook in another way.

The answer might be large, so print it modulo 998244353998244353 .

Input

The only line of the input contains two integers nn and kk ( 1n2000001 \le n \le 200000 ; 0kn(n1)20 \le k \le \frac{n(n - 1)}{2} ).

Output

Print one integer — the number of ways to place the rooks, taken modulo 998244353998244353 .

Samples

3 2
6
3 3
0
4 0
24
1337 42
807905441