#include <atcoder/modint>
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using mint = atcoder::modint998244353;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

void solve() {
  ll m, n;
  cin >> m >> n;
  mint i2 = mint(2).inv(), i3 = mint(3).inv();
  auto f = [&](mint x) -> mint {
    return x * x * x * i3 + x * x * i2 + x * i2 * i3;
  };
  mint ans = 0;
  ll p = 0;
  rep(ni, n) {
    ll x;
    cin >> x;
    ans += f(mint(x - p - 1));
    p = x;
  }
  ans += f(mint(m + 1 - p - 1));
  cout << ans.val() << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}