这个题解错了网上还没有题解 囧

fi = (i%2)? 1 : -1
Fn = (n+m−1m−1) - ∑ni=0 ∑m−1j=1 (n−i∗(j+1)+m−2 m−2) * (m−1 j)*fj
Fn *= m

#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

const int MAXN = 1e5+5;

const int MOD = 998244353;

ll C[1005][1005];

int main(){

    int n,m;

    C[0][0] = 1;

    for(int i = 1; i <= 1000; ++i) {

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

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

            C[i][j] = (C[i-1][j-1]+C[i-1][j]) %MOD;

        }

    }

    while(~scanf("%d %d",&n,&m)) {

        ll ans = C[n+m-1][m-1];

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

            int fl = -1;

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

                if(n-i*(j+1) < 0) continue;

                int tt = n-i*(j+1);

                ans = (ans + fl*C[tt+m-2][m-2]*C[m-1][j]%MOD + MOD) %MOD;

                fl *= -1;

            }

        }

        printf("%lld\n",ans*m%MOD);

    }

    return 0;

}