#include<bits/stdc++.h>
using namespace std;
#define endl '\n'
#define X first
#define Y second
#define umap unordered_map
using ll = long long;
using ull = unsigned long long;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int maxn = 2e5 + 5, mod = 1e9 + 7, maxp = 31, inf = 1e9;
const ll INF = 1e18;
const double eps = 1e-6;

void solve() {
    ll n, m;
    cin >> n >> m;
    map<ll, int> f;
    for (int i = 0; i < m; i++) {
        ll x;
        cin >> x;
        f[x] = 1;
    }

    int ans = 0;

    ll len = n;
    while (len > 1) {
        map<ll, int> g;
        for (auto [x, y] : f) {
            int p = min(x, len + 1 - x);
            if (f[x] == f[len + 1 - x]) {
                g[p] = f[x];
                continue;
            }
            if (f[x] == 2 || f[len + 1 - x] == 2) {
                g[p] = min(f[x], f[len + 1 - x]);
            }
            else {
                // cout << x << endl;
                ans++;
                f[x] = f[len + 1 - x] = 2;
                g[p] = 2;
            }
        }

        f = g;
        len /= 2;
    }

    cout << ans << endl;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int tt = 1;
    // cin >> tt;
    while (tt--) solve();
    return 0;
}