#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
constexpr int MOD = 998244353;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, l, r) for (int i = (l); i < (int)(r); i++)
#define all(v) v.begin(), v.end()
void ch(ll &a,ll b){
  a=(a+b)%MOD;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  ll N,M;
  cin>>M>>N;
  auto sum3=[](ll a){
    a%=MOD;
    return a*(a+1)%MOD*(a*2+1)%MOD*166374059%MOD;
  };
  ll l=1,ans=0;
  rep(i,N){
    ll r;
    cin>>r;
    ch(ans,sum3(r-l));
    l=r+1;
  }
  ch(ans,sum3(M+1-l));
  ch(ans,MOD);
  cout<<ans<<'\n';
}