#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)s; i < (int)e; ++i)
#define all(a) (a).begin(), (a).end()

const ll mod = 998244353ll;

ll modpow(ll a, ll n, ll m) {
    ll res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % m;
        a = a * a % m;
        n >>= 1;
    }
    return res;
}

ll modinv(ll a, ll m) {
    return modpow(a, m - 2, m);
}

int main() {
    cin.tie(nullptr);
    
    ll M, N;
    cin >> M >> N;
    
    if (N == 0) {
        cout << M * (M + 1) % mod * (2 * M + 1) % mod * modinv(6, mod) % mod;
        return 0;
    }
    
    vector<ll> X(N);
    rep(i, 0, N) cin >> X[i];
    
    ll ans = 0;
    ll now = 1;
    ll k;
    rep(i, 0, N) {
        k = X[i] - now;
        ans += k * (k + 1) % mod * (2 * k + 1) % mod * modinv(6, mod) % mod;
        ans %= mod;
        now = X[i] + 1;
    }
    k = M - X[N - 1];
    ans += k * (k + 1) % mod * (2 * k + 1) % mod * modinv(6, mod) % mod;
    ans %= mod;
    
    cout << ans << '\n';
}