#include"atcoder/modint"
#include<bits/stdc++.h>
using namespace std;
int main(){
  cin.tie(0)->sync_with_stdio(0);
  using mint=atcoder::modint998244353;
  using ll=long long;
  ll M;int N;cin>>M>>N;
  vector<ll>X(N+2);for(int i=1;i<=N;++i)cin>>X[i];
  X[N+1]=M+1;
  mint ans;
  mint inv6=mint::raw(6).inv();
  for(int i=0;i<=N;++i){
    mint x=mint(X[i+1]-X[i]-1);
    ans+=x*(x+mint::raw(1))*(mint::raw(2)*x+mint::raw(1))*inv6;
  }
  cout<<ans.val()<<endl;
}