#include<bits/stdc++.h>
using namespace std;
#include <atcoder/all> 
using mint = atcoder::modint998244353;
#define int long long
signed main() {
  int n,m;
  cin >> n >> m;
  vector<int> a(m+2);
  a[0] = 0;
  for(int i = 1; i <= m; i++) cin >> a[i];
  a[m+1] = n+1;
  mint ans = 0, mod = 998244353;
  for(int i = 0; i < m+1; i++) {
    mint s = a[i+1]-a[i]-1;
    ans+=s*(s+1)*(s*2+1);
  }
  ans/=6;
  cout << ans.val() << endl;
}