import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		long m = Long.parseLong(sa[0]);
		int n = Integer.parseInt(sa[1]);
		sa = br.readLine().split(" ");
		long[] x = new long[n + 2];
		for (int i = 0; i < n; i++) {
			x[i + 1] = Long.parseLong(sa[i]);
		}
		br.close();
		x[n + 1] = m + 1;

		int mod = 998244353;
		long m6 = modinv(6, mod);
		long ans = 0;
		for (int i = 1; i < x.length; i++) {
			long d = x[i] - x[i - 1] - 1;
			long val = d * (d + 1) % mod * (2 * d + 1) % mod * m6 % mod;
			ans += val;
		}
		ans %= mod;
		System.out.println(ans);
	}

	static long modinv(long a, int m) {
		long b = m;
		long u = 1;
		long v = 0;
		long tmp = 0;

		while (b > 0) {
			long t = a / b;
			a -= t * b;
			tmp = a;
			a = b;
			b = tmp;

			u -= t * v;
			tmp = u;
			u = v;
			v = tmp;
		}

		u %= m;
		if (u < 0) u += m;
		return u;
	}
}