#include <bits/stdc++.h>   using namespace std;   const int N = 300005, T = 100000;   int n, m, a[N], l[N], r[N];   int main() {     ios::sync_with_stdio(false);     cin >> n >> m;     for (int i = 1; i <= m; i++) cin >> a[i];     if (n == 1) {       cout << 0 << endl;       return 0;     }     for (int i = m; i > 0; --i) {       l[a[i] + T + i] = l[a[i] + T + i + 1] + 1;       r[a[i] + T - i] = r[a[i] + T - i - 1] + 1;     }     long long ans = 0;     for (int i = 1; i <= n; i++)       ans += min(n, i + (m + 1) - r[i + T]) - max(1, i - (m + 1) + l[i + T]) + 1;     cout << ans << endl;     return 0;   }   