#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9 + 10;
const ll INFL = 4e18;

#include <atcoder/modint>
using mint = atcoder::modint998244353;

int main() {
    ll M;
    int N;
    cin >> M >> N;
    vector<ll> X(N);
    for (int i = 0; i < N; i++) cin >> X[i];
    X.insert(X.begin(), 0);
    X.push_back(M + 1);

    mint ans = 0;
    mint inv = mint::raw(6).inv();
    for (int i = 0; i <= N; i++) {
        mint d = X[i + 1] - X[i] - 1;
        ans += d * (d + 1) * (d * 2 + 1) * inv;
    }

    cout << ans.val() << endl;
}