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

using ll = long long;
constexpr int mod = 998244353, inv6 = (mod + 1) / 6;

ll csum(ll N) {
	N %= mod;
	return N * (N + 1) % mod * (2 * N + 1) % mod * inv6 % mod;
}

int main(void) {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	
	ll M; int N;
	cin >> M >> N;
	
	vector<ll> X(N);
	for(int i = 0; i < N; ++i) cin >> X[i];
	
	X.insert(X.begin(), 0);
	X.push_back(M + 1);
	
	ll ans = 0;
	for(int i = 0; i <= N; ++i) ans = (ans + csum(X[i + 1] - X[i] - 1)) % mod;
	
	cout << ans << "\n";
	
	return 0;
}