#include <bits/stdc++.h>
#include <atcoder/modint>

using namespace std;
using ll = long long;
using namespace atcoder;
using mint = modint998244353;

int main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    ll M, N, p=0, X;
    mint Y, ans=0, iv=mint(6).inv();
    cin >> M >> N;
    M++;

    for (int i=0; i<N; i++){
        cin >> X;
        Y = X-p-1;
        ans += Y*(Y+1)*(Y*2+1)*iv;
        p = X;
    }

    Y = M-p-1;
    ans += Y*(Y+1)*(Y*2+1)*iv;
    cout << ans.val() << endl;

    return 0;
}