import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Main {
    static final int MOD = 998244353;
    static final long DIV6 = pow(6, MOD - 2);
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        long m = sc.nextLong();
        int n = sc.nextInt();
        long ans = 0;
        long prev = 0;
        while (n-- > 0) {
            long x = sc.nextLong();
            ans += calc(x - prev - 1);
            ans %= MOD;
            prev = x;
        }
        ans += calc(m - prev);
        ans %= MOD;
        System.out.println(ans);
    }
    
    static long calc(long x) {
        if (x >= MOD) {
            return calc(x % MOD);
        } else {
            return x * (x + 1) % MOD * (2 * x % MOD + 1) % MOD * DIV6 % MOD;
        }
    }
    
    static long pow(long x, int p) {
        if (p == 0) {
            return 1;
        } else if (p % 2 == 0) {
            return pow(x * x % MOD, p / 2);
        } else {
            return pow(x, p - 1) * x % MOD;
        }
    }
}

class Scanner {
    BufferedReader br;
    StringTokenizer st = new StringTokenizer("");

    public Scanner() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            
        }
    }
    
    public int nextInt() {
        return Integer.parseInt(next());
    }
    
    public long nextLong() {
        return Long.parseLong(next());
    }
    
    public double nextDouble() {
        return Double.parseDouble(next());
    }
    
    public String next() {
        try {
            while (!st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return st.nextToken();
        }
    }
    
}