#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace std;
using mint=atcoder::modint998244353;

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