#include "bits/stdc++.h"

using namespace std;

#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

using ll = long long;
using LL = long long;
using pii = pair<int, int>;

LL n, m;
LL ans;

void f(LL n, vector<pair<LL, int>> &v) {
	if (n == 1) return;
	
	vector<pair<LL, int>> v2;
	LL n2 = n / 2;
	int p1 = 0;
	int p2 = (int)v.size() - 1;
	while (p1 <= p2) {
		LL dis1 = n2 - v[p1].first;
		LL dis2 = v[p2].first - (n2 + 1);
		if (dis1 == dis2) {
			if (v[p1].second == v[p2].second) {
				v2.push_back(v[p1]);
			}
			else if (v[p1].second == 2) {
				v2.push_back({v[p1].first, v[p2].second});
			}
			else if (v[p2].second == 2) {
				v2.push_back(v[p1]);
			}
			else {
				v2.push_back({v[p1].first, 2});
				ans++;
			}
			p1++;
			p2--;
		}
		else if (dis1 > dis2) {
			if(v[p1].second == 1){
				ans++;
				v2.push_back({v[p1].first, 2});
			}
			else if (v[p1].second == 2) {
				v2.push_back({v[p1].first, 0});
			}
			else {

			}
			p1++;
		}
		else {
			LL pl = n2 - dis2;
			if (v[p2].second == 1) {
				ans++;
				v2.push_back({pl, 2});
			}
			else if (v[p2].second == 2) {
				v2.push_back({pl, 0});
			}
			else {

			}
			p2--;
		}
	}
	f(n2, v2);
}
void solve(int TC) {
	cin >> n >> m;
	vector<pair<LL, int>> v;
	ans = 0;
	for (int i = 1; i <= m; i++) {
		LL tmp;
		cin >> tmp;
		v.push_back({tmp, 1});
	}
	f(n, v);
	cout << ans << "\n";
	return;
}

int32_t main() {
	cin.tie(0)->sync_with_stdio(0);
	cout << fixed << setprecision(10);

	int t = 1;
	//cin >> t;
	for (int i = 1; i <= t; i++) {
		solve(i);
	}
}