#include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define REP(i,n) for(int i=0;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1<<29
#define LINF LLONG_MAX/3
#define MP make_pair
#define PB push_back
#define pb push_back
#define EB emplace_back
#define ALL(v) (v).begin(),(v).end()
#define all(v) ALL(v)
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
#define CININIT cin.tie(0),ios::sync_with_stdio(false)
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T> &st){ os << "["; for(auto a : st) cout << a << ","; os << "]"; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

int N,M;
vector<bool> ex;

int main(){
    cin >> N >> M;
    ex = vector<bool>(N,true);
    rep(i,M){
        int a;
        cin >> a;
        ex[a] = false;
    }

    int NN = N-1;
    vector<int> tree(N+10,-1);
    int ans=0;
    
    for(int i=NN-(N/2),j=0;i<NN;i++,j+=2){
        int l=j;
        int r=j+1;
        if(ex[l]==false and ex[r]==false){
            continue;
        }
        else if(ex[l]==false or ex[r]==false){
            tree[i] = 1;
        }
        else{
            tree[i] = 1;
            ans++;
        }
    }

    for(int i=NN-(N/2)-1;i>=0;i--){
        int l = i*2+1;
        int r = i*2+2;
//        cerr << i << " " << l << " " << r << endl;
        if(tree[l]==-1 and tree[r]==-1){
            tree[i] = -1;
        }
        else if(tree[l]==-1 or tree[r]==-1){
            tree[i] = 1;
        }
        else{
            tree[i] = 1;
            ans++;
        }
    }


    //rep(i,NN){
    //    cout << i << " " << tree[i] << endl;
    //}

    cout << ans << endl;
    return 0;
}