牛客竞赛题目讲解_Symmetric Matrix

// https://ac.nowcoder.com/acm/contest/20322/B

#include<bits/stdc++.h>

#define ll long long

int n,m;

ll f[100050];

int main(){

    f[1]=0;f[2]=f[3]=1;

    while(std::cin>>n>>m){

        for(ll i=4;i<=n;i++)

            f[i]=(((i-1)*(f[i-1]+f[i-2])-(i-1)*(i-2)/2*f[i-3]))%m;

        printf("%lld\n",(f[n]+m)%m);       

    }

    return 0;

}