9161: ABC281 —— Ex - Alchemy
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Description
Takahashi has $A$ kinds of level-$1$ gems, and $10^{10^{100}}$ gems of each of those kinds.
For an integer $n$ greater than or equal to $2$, he can put $n$ gems that satisfy all of the following conditions into a cauldron to generate a level-$n$ gem in return.
- No two gems are of the same kind.
- Every gem's level is less than $n$.
- For every integer $x$ greater than or equal to $2$, there is at most one level-$x$ gem.
Find the number of kinds of level-$N$ gems that Takahashi can obtain, modulo $998244353$.
Here, two level-$2$ or higher gems are considered to be of the same kind if and only if they are generated from the same set of gems.
- Two sets of gems are distinguished if and only if there is a gem in one of those sets such that the other set does not contain a gem of the same kind.
Any level-$1$ gem and any level-$2$ or higher gem are of different kinds.
For an integer $n$ greater than or equal to $2$, he can put $n$ gems that satisfy all of the following conditions into a cauldron to generate a level-$n$ gem in return.
- No two gems are of the same kind.
- Every gem's level is less than $n$.
- For every integer $x$ greater than or equal to $2$, there is at most one level-$x$ gem.
Find the number of kinds of level-$N$ gems that Takahashi can obtain, modulo $998244353$.
Here, two level-$2$ or higher gems are considered to be of the same kind if and only if they are generated from the same set of gems.
- Two sets of gems are distinguished if and only if there is a gem in one of those sets such that the other set does not contain a gem of the same kind.
Any level-$1$ gem and any level-$2$ or higher gem are of different kinds.
Input
The input is given from Standard Input in the following format:
```
$N$ $A$
```
```
$N$ $A$
```
Output
Print the answer.
Constraints
- $1 \leq N \leq 2 \times 10^5$
- $1 \leq A \leq 10^9$
- $N$ and $A$ are integers.
- $1 \leq A \leq 10^9$
- $N$ and $A$ are integers.
Sample 1 Input
3 3
Sample 1 Output
10
Here are ten ways to obtain a level-3 gem.
-
Put three kinds of level-1 gems into the cauldron.
- Takahashi has three kinds of level-1 gems, so there is one way to choose three kinds of level-1 gems. Thus, he can obtain one kind of level-3 gem in this way.
-
Put one kind of level-2 gem and two kinds of level-1 gems into the cauldron.
-
A level-2 gem can be obtained by putting two kinds of level-1 gems into the cauldron.
- Takahashi has three kinds of level-1 gems, so there are three ways to choose two kinds of level-1 gems. Thus, three kinds of level-2 gems are available here.
- There are three kinds of level-2 gems, and three ways to choose two kinds of level-1 gems, so he can obtain 3×3=9 kinds of level-3 gems in this way.
-
A level-2 gem can be obtained by putting two kinds of level-1 gems into the cauldron.
Sample 2 Input
1 100
Sample 2 Output
100
Sample 3 Input
200000 1000000000
Sample 3 Output
797585162
HINT
相同题目:ABC281。