#include <bits/stdc++.h>

typedef long long ll;

std::map<ll, char> mp;

int ans;

void work(ll n) {
    while (n > 1) {
        std::map<ll,char> np;
        for (auto &[i, x]: mp) {
            char t1, t2;
            if (mp.count(i)) {
                t1 = mp[i];
            } else {
                t1 = 0;
            }
            if (mp.count(n - i - 1)) {
                t2 = mp[n - i - 1];
            } else {
                t2 = 0;
            }
            if (i < n / 2) {
                if (t1 == t2) {
                    np[i] = t1;
                } else if (t1 == 2) {
                    np[i] = t2;
                } else if (t2 == 2) {
                    np[i] = t1;
                } else {
                    ++ans;
                    np[i] = 2;
                }
            } else if (!t2) {
                if (t1 == t2) {
                    np[n - i - 1] = t1;
                } else if (t1 == 2) {
                    np[n - i - 1] = t2;
                } else if (t2 == 2) {
                    np[n - i - 1] = t1;
                } else {
                    ++ans;
                    np[n - i - 1] = 2;
                }
            }
        }
        mp = np;
        n /= 2;
    }
}

int main() {
    //freopen("in", "r", stdin);
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    ll n;
    int m;
    std::cin >> n >> m;
    for (int i = 1; i <= m; ++i) {
        ll x;
        std::cin >> x;
        x--;
        mp[x] = 1;
    }
    work(n);
    std::cout << ans << std::endl;
    return 0;
}