#include <bits/stdc++.h>
#include <math.h>
			
using namespace std;

// c++ short types
#define Int long long
#define vt vector
typedef unsigned long long ull;

const int mod = 1e9 + 123, base = 517;

void solver() {
	int n, m; cin >> n >> m;
	vt<int> a;
	int prev = 0;
	for(int i = 0; i < m; ++i) {
		int x; cin >> x;
		a.push_back(x-prev-1);
		prev = x;
	}
	a.push_back(n-prev);
	vt<pair<int, ull>> pw(m+2, {1, 1});
	for(int i = 1; i < pw.size(); ++i) {
		pw[i].first = pw[i-1].first * 1ll * base % mod;
		pw[i].second = pw[i-1].second * base;
	}
	vt<vt<pair<int, ull>>> b(m+2, vt<pair<int, ull>>(m+2, {0, 0}));
	for(int i = 0; i < a.size(); ++i) {
		for(int j = i; j < a.size(); ++j) {
			if(i != j) {
				b[i][j] = b[i][j-1];
			}
			b[i][j].first = (b[i][j].first + pw[j-i].first * 1ll * a[j] % mod) % mod;
			b[i][j].second = b[i][j].second + pw[j-i].second * a[j];
		}
	}
	Int res = *max_element(a.begin(), a.end());
	auto Calc = [](vt<pair<int, int>> &z) {
		z.push_back({-1, 0});
		sort(z.rbegin(), z.rend());
		Int res = 0, maxb = z[0].second;
		for(int i = 0; i < z.size(); ++i) {
			if(i) {
				res += (maxb+1) *  (z[i-1].first - z[i].first);
			}
			maxb = max(maxb, (Int)z[i].second);
		}
		return res;
	};
	{
		vt<pair<int, int>> z;
		for(int i = 0; i + 1 < a.size(); ++i) {
			z.push_back({a[i], a[i+1]});
		}
		res += Calc(z);
	}
	for(int le = 0; le < m + 2; ++le) {
		vt<pair<pair<int, ull>, pair<int, int>>> v;
		for(int i = 1; i + le + 1 < a.size(); ++i) {
			v.push_back({b[i][i+le], {a[i-1], a[i+le+1]}});
		}
		sort(v.begin(), v.end());
		for(int i = 0; i < v.size(); ++i) {
			int j = i; 
			vt<pair<int, int>> z;
			while(j < v.size() && v[j].first == v[i].first) {
				z.push_back(v[j].second);
				++j;
			}
			// (ai, bi)
			reverse(z.begin(), z.end());
			res += Calc(z);
			i = j -1;
		}
	}
	cout << res << "\n";
}

int main()
{
	ios::sync_with_stdio(false); cin.tie(nullptr);
	int tt = 1;
	for(int t = 0; t < tt; ++t) {
    solver();
	}
	return 0;
}