#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
using ll = long long;
#include <atcoder/modint>
using namespace atcoder;
#define rep(i, n) for (int i = 0; i< (int)(n); i++) 

using mint = modint998244353;

mint solve(ll a){
    mint res = a; res *= (a+1); res *= (2*a + 1);
    res /= (mint)6;
    return res;  
}

int main(){
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    ll m; int n; cin >> m >> n;
    if(n == 0){
        mint ans = m; ans *= (m+1); ans *= (2*m + 1);
        ans /= (mint)6;
        cout << ans.val() << endl;
        return 0;
    }

    vector<ll> x(n); rep(i, n)cin >> x[i];
    mint score = (mint)0;
    rep(i, n){
        if(i == 0){
            score += solve(x[i] - 1);
            continue;
        }
        score += solve(x[i] - x[i-1] - 1);
    }
    score += solve(m - x[n-1]);
    cout << score.val() << endl;

    return 0;
}