#include <bits/stdc++.h>

using namespace std;


#define ll long long
#define flt double
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, n) for(int i = 0; i < n; i += 1)
#define len(a) ((int)(a).size())


const ll inf = 1e18;
mt19937 rnd(234);


int32_t main() {
    if (1) {
        ios::sync_with_stdio(0);
        cin.tie(0);
        cout.tie(0);
    }
    ll n, m;
    cin >> n >> m;
    vector<pair<ll, int>> a;
    rep(i, m) {
        ll p;
        cin >> p;
        --p;
        a.push_back(make_pair(p, 1));
    }
    ll moves = 0;
    while (n > 1) {
        assert(n % 2 == 0);
        n /= 2;
        vector<pair<ll, int>> b, c;
        for (auto [p, x] : a) {
            if (p < n) {
                b.push_back(make_pair(p, x));
            } else {
                c.push_back(make_pair(2 * n - p - 1, x));
            }
        }
        reverse(all(c));
        a.clear();
        int i = 0;
        int j = 0;
        while (i < len(b) or j < len(c)) {
            if (i < len(b) and j < len(c) and b[i].first == c[j].first) {
                if (b[i].second == 2) {
                    a.push_back(c[i]);
                } else if (c[i].second == 2) {
                    a.push_back(b[i]);
                } else {
                    assert(b[i] == c[i]);
                    a.push_back(b[i]);
                }
                ++i;
                ++j;
                continue;
            }
            if (j == len(c) or (i < len(b) and b[i].first < c[j].first)) {
                if (b[i].second == 1) {
                    moves += 1;
                    a.push_back(make_pair(b[i].first, 2));
                }
                ++i;
            } else {
                if (c[j].second == 1) {
                    moves += 1;
                    a.push_back(make_pair(c[j].first, 2));
                }
                ++j;
            }
        }
    }
    cout << moves << "\n";
    return 0;
}