#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using ll = unsigned long long;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll m, pre = 0, v;
    size_t n;
    cin >> m >> n;
    atcoder::modint998244353 ans, len;
    for(size_t i = 0; i < n; i++){
        cin >> v;
        len = v - pre - 1;
        pre = v;
        ans += len * (len + 1) * (len + len + 1);
    }
    len = m - pre;
    ans += len * (len + 1) * (len + len + 1);
    cout << (ans * 166374059).val() << '\n';
}