#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using ll = long long;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll m, n;
    cin >> m >> n;
    vector<ll> x(n + 2);
    x[n + 1] = m + 1;
    for(int i = 1; i <= n; i++) cin >> x[i];
    atcoder::modint998244353 ans, v;
    for(int i = 0; i <= n; i++){
        v = x[i + 1] - x[i] - 1;
        ans += v * (v + 1) * (2 * v + 1);
    }
    cout << (ans / 6).val() << '\n';
}