Score : $200$ points

问题描述

Takahashi 有 $N$ 个朋友。他们的昵称分别是：朋友 $1$、朋友 $2$、$\ldots$、朋友 $N$。

有一天，Takahashi 不小心让他的一个朋友——朋友 $X$ 知道了他的一个羞耻的秘密。
对于每个 $i = 1, 2, \ldots, N$，当朋友 $i$ 知道了这个秘密后，若朋友 $A_i$ 还未知道这个秘密，他/她将会把这个秘密分享给朋友 $A_i$。

最终会有多少个 Takahashi 的朋友知道这个秘密？

以上为通义千问 qwen-max 翻译，仅供参考。

Problem Statement

Takahashi has $N$ friends. They have nicknames: Friend $1$, Friend $2$, $\ldots$, Friend $N$.

One day, Takahashi accidentally let one of his friends, Friend $X$, learn his shameful secret.
For each $i = 1, 2, \ldots, N$, when Friend $i$ learns the secret, he/she will share it with Friend $A_i$, if Friend $A_i$ has not already learned it.

How many of Takahashi's friends will learn the secret in the end?

Constraints

• $2 \leq N \leq 10^5$
• $1 \leq X \leq N$
• $1 \leq A_i \leq N$
• $A_i \neq i$
• All values in input are integers.

Input

Input is given from Standard Input in the following format:

$N$ $X$

$A_1$ $A_2$ $\cdots$ $A_N$

Output

Print the answer.

Sample Input 1

4 2
3 1 1 2

Sample Output 1

3

Takahashi's secret will be learned by Friend $1$, Friend $2$, and Friend $3$, as follows.

• One day, Takahashi let Friend $2$ learn the secret.
• Friend $2$ shares it with Friend $1$.
• Friend $1$ shares it with Friend $3$.

In the end, three of his friends learn the secret, so we print $3$.

Sample Input 2

20 12
7 11 10 1 7 20 14 2 17 3 2 5 19 20 8 14 18 2 10 10

Sample Output 2

7

update @ 2024/3/10 09:58:16