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

#define For(i, a, b) for(int i = (a); i < (b); i++)
#define rep(i, n) For(i, 0, n)
#define rFor(i, a, b) for(int i = (a); i >= (b); i--)
#define ALL(v) (v).begin(), (v).end()
#define rALL(v) (v).rbegin(), (v).rend()

using lint = long long;
using ld = long double;

int INF = 2000000000;
lint LINF = 1000000000000000000;

struct SetupIo {
    SetupIo() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
        cout << fixed << setprecision(15);
    }
} setupio;

#include <atcoder/modint>
using mint = atcoder::modint998244353;

int main() {
    lint m, n;
    cin >> m >> n;
    vector<lint> x(n + 2);
    x[0] = 0;
    x[n + 1] = m + 1;
    For(i, 1, n + 1) {
        cin >> x[i];
    }
    mint ans = 0;
    rep(i, n + 1) {
        lint cnt = x[i + 1] - x[i] - 1;
        ans += mint(cnt) * mint(cnt + 1) * mint(2 * cnt + 1) / mint(6);
    }
    cout << ans.val() << "\n";
}