#include <bits/stdc++.h>
using namespace std;
#include <atcoder/modint>
using namespace atcoder;
using ll = long long;
using P = pair<ll,ll>;
#define fix(x) fixed << setprecision(x)
#define asc(x) x, vector<x>, greater<x>
#define rep(i, n) for(ll i = 0; i < n; ++i)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL << 62), MOD = 998244353;
constexpr int INF = (1 << 30);
using mint = modint998244353;
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    ll m,n;
    cin >> m >> n;
    mint pre = 0, ans = 0;
    rep(i,n+1){
        ll a;
        if(i<n) cin >> a;
        else a = m+1;
        mint x = a - pre - 1;
        ans += x * (x+1) * (2*x+1) / 6;
        pre = a;
    }
    cout << ans.val() << '\n';
    return 0;
}