#include <bits/stdc++.h>
using namespace std;

void fast_io() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
}

#include <atcoder/modint>
using mint = atcoder::modint998244353;
int main() {
	fast_io();
	long long m, n;
	cin >> m >> n;
	vector<long long> x(n + 2);
	x[0] = 0;
	for (int i = 1; i <= n; i++) {
		cin >> x[i];
	}
	x[n + 1] = m + 1;
	mint ans = 0;
	for (int i = 1; i <= n + 1; i++) {
		mint d = x[i] - x[i - 1] - 1;
		ans += d * (d + 1) * (2 * d + 1) / 6;
	}
	cout << ans.val() << endl;
}