using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (m, n) = (c[0], c[1]);
        if (n == 0)
        {
            ReadLine();
            WriteLine(Score(m));
            return;
        }
        var x = NList;
        var ans = 0L;
        for (var i = 0; i < n; ++i)
        {
            if (i == 0) ans = (ans + Score(x[i] - 1)) % mod;
            else ans = (ans + Score(x[i] - x[i - 1] - 1)) % mod;
        }
        ans = (ans + Score(m - x[^1])) % mod;
        WriteLine(ans);
    }
    static int mod = 998_244_353;
    static int pa = 166_374_059;
    static long Score(long cmb)
    {
        cmb %= mod;
        return cmb * (cmb + mod + 1) % mod * (cmb * 2 + 1) % mod * pa % mod;
    }
}