#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using ll = long long;
int main () {
	ll M, N;
	cin >> M >> N;
	std::vector<ll> A = {0};
	for (int i = 0; i < N; i ++) {
		ll a;
		cin >> a;
		A.push_back(a);
	}
	A.push_back(M + 1);
	mint ans = 0;
	for (int i = 0; i <= N; i ++) {
		mint d = A[i + 1] - A[i];
		ans += d * (d + 1) * (2 * d + 1) / 6;
	}
	cout << ans.val() << endl;
}