#include <bits/stdc++.h>
#define rep(i, a, n) for (int i = a; i < (int)(n); i++)
#define all(a) begin(a), end(a)
#define rall(a) rbegin(a), rend(a)
#define vi vector<int>
#define vs vector<string>
#define vvi vector<vector<int>>
#define vb vector<bool>
#define ll long long
#define dd double
using namespace std;

void faster()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
}

template <typename T>
void input(T &x)
{
    cin >> x;
}
template <typename T, typename... Args>
void input(T &x, Args &...args)
{
    cin >> x;
    input(args...);
}

template <typename T>
void sorts(vector<T> &vec)
{
    sort(vec.begin(), vec.end());
}

template <typename T>
void rsorts(vector<T> &vec)
{
    sort(vec.begin(), vec.end(), greater<T>());
}

template <class T>
inline bool chmin(T &a, T b)
{
    if (a > b)
    {
        a = b;
        return true;
    }
    return false;
}
template <class T>
inline bool chmax(T &a, T b)
{
    if (a < b)
    {
        a = b;
        return true;
    }
    return false;
}

struct timekeeper
{
    chrono::high_resolution_clock::time_point starttime;
    chrono::high_resolution_clock::time_point endtime;
    chrono::duration<double> duration;
};
unsigned ll sum_1_to_n(const unsigned ll &a)
{
    return a * (a + 1) / 2;
}

ll rounding(ll target, ll keta)
{
    ll now = 1;
    rep(i, 0, keta)
    {
        target /= now;
        int k = target % 10;
        if (k >= 5)
        {
            target += 10 - k;
        }
        else
        {
            target -= k;
        }
        target *= now;
        now *= 10;
    }
    return target;
}
string RLE(const string &s)
{
    int cnt = 0;
    char ch = s[0];
    string res;
    for (char now : s)
    {
        if (now == ch)
        {
            cnt++;
        }
        else
        {
            if (cnt > 1)
            {
                res += to_string(cnt) + ch;
            }
            else
            {
                res += ch;
            }
            ch = now;
            cnt = 1;
        }
    }
    if (cnt > 1)
    {
        res += to_string(cnt) + ch;
    }
    else
    {
        res += ch;
    }
    return res;
}

unsigned ll power(const unsigned ll &n)
{
    unsigned ll a = n * (n + 1) / 2;
    unsigned ll b = 2 * n + 1;
    return a * b / 3;
}
// 上、下、右、左、右上、右下、左上、左下
vi xmove = {0, 0, 1, -1, 1, 1, -1, -1};
vi ymove = {-1, 1, 0, 0, -1, 1, -1, 1};

const unsigned long long MOD = 998244353;

int main()
{
    unsigned long long m, n;
    cin >> m >> n;
    vector<unsigned long long> vl(n);

    for (unsigned long long i = 0; i < n; ++i)
    {
        cin >> vl[i];
    }

    if (n == 0)
    {
        cout << power(m) << endl;
        return 0;
    }
    unsigned ll ans = 0;
    ans += power(vl[0] - 1) % 998244353;
    unsigned ll note = vl[0] + 1;
    rep(i, 1, n)
    {
        ans += power(vl[i] - note) % 998244353;
        note = vl[i] + 1;
    }
    ans += power(m - note + 1) % 998244353;
    cout << ans % 998244353 << endl;
}