#P3200. Counting Interesting Permutations
Counting Interesting Permutations
Counting Interesting Permutations
Given a positive integer \(n\), we define a sequence of length \(2n\) as interesting if and only if it satisfies the following conditions:
- The sequence is a permutation of the set \(\{1,2,\dots,2n\}\).
- If the sequence is \(\{a_1, a_2, \dots, a_{2n}\}\), then the subsequence of all odd-indexed terms \(a_1,a_3,\dots,a_{2n-1}\) is in strictly increasing order and the subsequence of all even-indexed terms \(a_2,a_4,\dots,a_{2n}\) is in strictly increasing order.
- For every \(i\) with \(1\le i\le n\), the adjacent pair \(\left(a_{2i-1}, a_{2i}\right)\) satisfies \(a_{2i-1} < a_{2i}\).
Your task is to compute the number of different interesting sequences of length \(2n\). Since the answer can be very large, output it modulo \(p\), where \(p\) is a given modulus.
Hint: It can be shown that the answer is the \(n\)th Catalan number, given by:
\[ C_n = \frac{1}{n+1}\binom{2n}{n}. \]inputFormat
The first and only line of input contains two integers \(n\) and \(p\) (separated by a space), where \(n\) is the half-length of the permutation and \(p\) is a prime number specifying the modulus.
outputFormat
Output a single integer: the number of interesting sequences modulo \(p\).
sample
1 1000000007
1