#include<bits/stdc++.h>

using namespace std;
#define IOS ios::sync_with_stdio(false); cin.tie(0);


using ll = long long;
#define endl "\n"

ll n, m;
set<ll> a1, a2;

void print() {
    for (int i = 1; i <= n; ++i) {
        if (a1.find(i) != a1.end())cout << 1;
        else if (a2.find(i) != a2.end())cout << 2;
        else cout << 0;
    }
    cout<<endl;
    for(auto&& i:a1){
        cout<<i<<" ";
    }
    cout<<endl;
    for(auto&& i:a2){
        cout<<i<<" ";
    }
    cout<<endl;
}


int main() {
    cin >> n >> m;
    for (ll i = 0; i < m; ++i) {
        ll x;
        cin >> x;
        a1.insert(x);
    }
    ll ans = 0;
    while (n != 1) {
        vector<ll> t;
//        print();
        for (auto i = a1.begin(); i != a1.end(); ++i) {
            if (*i > n)break;
            auto r = n + 1 - *i;
            if (a1.find(r) != a1.end())continue;
            if (auto ia2 = a2.find(r);ia2 != a2.end()) {
                t.emplace_back(*ia2);
                a2.erase(ia2);
            } else {
                ans++;
                a2.insert(*i);
                a2.insert(r);
                i = a1.erase(i);
            }
        }
        a1.insert(t.begin(), t.end());
        for (auto i = a2.begin(); i != a2.end(); ++i) {
            if (*i > n)break;
            auto r = n + 1 - *i;
            if (a2.find(r) != a2.end())continue;
            i = a2.erase(i);
        }
        n >>= 1;
    }
//    print();
    cout << ans;
}