#include <bits/stdc++.h>

#define lsb(x) (x & (-x))

using ull = unsigned long long;
using ll = long long;

using namespace std;



int main() {
#ifdef HOME
    ifstream cin("input.in");
    ofstream cout("output.out");
#endif
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    ll n;
    int m;
    cin >> n >> m;

    unordered_map<ll, int> values;
    for (int i = 0; i < m; i++) {
        ll pos;
        cin >> pos;
        values[pos] = 1;
    }
    
    int answer = 0;
    for (ll len = n / 2; len >= 1; len /= 2) {
        set<pair<ll, ll>> vis;
        for (auto [pos, v] : values) {
            if (pos > 2 * len) continue;

            ll a, b;
            if (pos <= len) {
                a = pos;
                b = 2 * len - pos + 1;
            } else {
                a = 2 * len - pos + 1;
                b = pos;
            }

            if (vis.find({a, b}) != vis.end()) continue;
            vis.insert({a, b});

            if (values[a] == values[b]) {
                continue;
            }

            if (values[a] == 2 || values[b] == 2) {
                values[a] = values[b] = min(values[a], values[b]);
                continue;
            }

            answer++;
            values[a] = values[b] = 2;
        }
    }

    cout << answer;

    return 0;
}