题意

题目链接

Sol

有点自闭，。我好像对组合数一窍不通(~~~~)

Orz shadowice

// luogu-judger-enable-o2

#include<bits/stdc++.h>

#define LL long long

int mod;

template<typename A, typename B> inline bool chmax(A &x, B y) {return x < y ? x = y, 1 : 0;}

template<typename A, typename B> inline bool chmin(A &x, B y) {return x > y ? x = y, 1 : 0;}

template <typename A, typename B> inline LL add(A x, B y) {if(x + y < 0) return x + y + mod; return x + y >= mod ? x + y - mod : x + y;}

template <typename A, typename B> inline void add2(A &x, B y) {if(x + y < 0) x = x + y + mod; else x = (x + y >= mod ? x + y - mod : x + y);}

template <typename A, typename B> inline LL mul(A x, B y) {return 1ll * x * y % mod;}

template <typename A, typename B> inline void mul2(A &x, B y) {x = (1ll * x * y % mod + mod) % mod;}

#define LL long long

using namespace std;

const int MAXN = 2001, INF = 1e9 + 10;

inline int read() {

    char c = getchar(); int x = 0, f = 1;

    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}

    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();

    return x * f;

}

LL N, fac[MAXN], C[MAXN][MAXN];

signed main() {

	N = read(); mod = read();

	fac[0] = 1;

	for(int i = 1; i <= N; i++) fac[i] = mul(i, fac[i - 1]);

	for(int i = 0; i <= N; i++) {

		C[i][0] = C[i][i] = 1;

		for(int j = 1; j < i; j++) C[i][j] = add(C[i - 1][j - 1], C[i - 1][j]);

	}

	LL res = 0;

	for(int i = 2; i <= N; i++) {

		LL ans = 0;

		for(int j = 1; j <= N - i + 1; j++) {

			add2(ans, 1ll * fac[j] % mod * C[N - i][j - 1] % mod * j % mod * (N - j) % mod * fac[N - j - 1] % mod * i % mod * (i - 1) % mod);

		}

		add2(res, ans);

	}

	cout << res;

    return 0;

}

/*

6

2 5 2 3 5 4

1

1

1

2

4

*/