#include <bits/stdc++.h>
using namespace std;
#define int long long
typedef pair<int, int> pii;
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, m;
    cin >> n >> m;
    map<int, pii> mp;
    for (int i = 1, x; i <= m; ++i) {
        cin >> x;
        mp[x] = pii(1, 0);
    }
    int ans = 0;
    while (n > 1) {
        int lim = n / 2;
        for (auto [x, _] : mp) {
            if (x > lim) break;
            if (!mp.count(n + 1 - lim)) mp[n + 1 - lim] = pii(0, 0);
        }
        while (!mp.empty() && prev(mp.end())->first > lim) {
            auto [pos, tmp] = *prev(mp.end());
            auto [val, is_sp] = tmp;
            mp.erase(--mp.end());
            if (is_sp) continue;
            int npos = n + 1 - pos;
            if (!mp.count(npos)) {
                ans += 1;
                mp[npos] = pii(0, 1); // not sure
                continue;
            }
            auto &[nval, nis_sp] = mp[npos];
            if (nis_sp) {
                nis_sp = 0;
                nval = val;
            } else if (nval == val) {
                // pass
            } else {
                ans += 1;
                nis_sp = 1;
            }
        }
        n = lim;
    }
    cout << ans << endl;
    return 0;
}