#include <algorithm>
#include <array>
#include <vector>
#include <utility>
#include <iostream>
#include <cassert>
#include <set>
#include <random>
#include <climits>
#include <map>
#include <queue>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
#define forn(i,n) for (int i = 0; i<n; i++)
#define forl(i,l,h) for (int i = l; i<h; i++)
#define el << '\n'
#define nl cout << '\n'
#define spc << ' '
#define pb push_back
#define all(x) x.begin(), x.end()
#define sz(x) (int) x.size()
#define fi first
#define se second
#define rsz resize
const int inf = 1000000000;
const ll inf2 = 2000000000000000000;

int main() {
    cin.tie(NULL); ios_base::sync_with_stdio(false);
    int n, m; cin >> n >> m;
    set<pii> pm, npm;
    forn(i,m) {
        int a; cin >> a;
        pm.insert({a-1, 1});
    }
    int tot = 0;
    while ((n = n/2)>0) {
        for (pii i : pm) {
            pii temp = {2*n-1 - i.fi, 0};
            auto ot = pm.lower_bound(temp);
            if (i.fi < n) {
                if (ot != pm.end() && (*ot).fi == temp.fi) {
                    if ((*ot).se == i.se) npm.insert(i);
                    else npm.insert({i.fi, 1});
                }
                else {
                    if (i.se==1) {
                        npm.insert({i.fi, 2});
                        tot++;
                    }
                }
            }
            else {
                if (ot != pm.end() && (*ot).fi != temp.fi) {
                    if (i.se==1) {
                        npm.insert({temp.fi, 2});
                        tot++;
                    }
                }
            }
        }
        swap(pm, npm);
        npm.clear();
    }
    cout << tot << endl;
}