#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(int i=0; i<int(n); i++)
const i64 INF = 1001001001001001001;
template<typename A> void chmin(A& l, const A& r){ if(r < l) l = r; }
template<typename A> void chmax(A& l, const A& r){ if(l < r) l = r; }
#include <atcoder/modint>
using Modint = atcoder::static_modint<998244353>;
using namespace std;

void testcase(){
    i64 M, N; cin >> M >> N;
    vector<i64> A(N+2);
    rep(i,N) cin >> A[i+1];
    A[N+1] = M+1;
    Modint ans;
    rep(i,N+1){
        i64 d = A[i+1] - A[i] - 1;
        ans += Modint(d) * Modint(d+1) * Modint(d*2+1) / 6;
    }
    cout << ans.val() << endl;
}

int main(){
    ios::sync_with_stdio(false); cin.tie(nullptr);
    testcase();
    return 0;
}