#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <set>
#include <map>
#include <cmath>
using namespace std;
typedef long long i64;
typedef long double ld;
typedef pair<i64,i64> P;
#define rep(i,s,e) for(int i = (s);i <= (e);i++)

/* include file*/
#include <functional>
#include <vector>
using namespace std;

template <class Monoid>
struct Segment {
  using Func = function<Monoid(Monoid, Monoid)>;

  vector<Monoid> node;
  Monoid ide;
  int n = 1;

  Func bin_f;
  Func update_f;

  Segment(const vector<Monoid>& init, Monoid ide_, Func f_, Func u_f)
      : bin_f(f_), ide(ide_), update_f(u_f) {
    int sz = init.size();
    while (n < sz) n *= 2;
    node.assign(n * 2 - 1, ide);
    for (int i = 0; i < sz; i++) node[i + n - 1] = init[i];
    for (int i = n - 2; i >= 0; i--)
      node[i] = bin_f(node[i * 2 + 1], node[i * 2 + 2]);
  }

  void update(int i, Monoid x) {
    i += n - 1;
    node[i] = update_f(node[i], x);
    while (i) {
      i = (i - 1) / 2;
      node[i] = bin_f(node[i * 2 + 1], node[i * 2 + 2]);
    }
  }

  Monoid get_inter(int a, int b, int k = 0, int l = 0, int r = -1) {
    if (r < 0) r = n;
    if (a <= l && r <= b) return node[k];
    if (r <= a || b <= l) return ide;

    Monoid lm = get_inter(a, b, k * 2 + 1, l, (l + r) / 2);
    Monoid rm = get_inter(a, b, k * 2 + 2, (l + r) / 2, r);
    return bin_f(lm, rm);
  }
};

// http://judge.u-aizu.ac.jp/onlinejudge/review.jsp?rid=2730414#1
// http://judge.u-aizu.ac.jp/onlinejudge/review.jsp?rid=2730425#1

int n;
int m;

int main(){
  cin >> n >> m;
  vector<int> on(n,0);
  for(int i = 0;i < m;i++){
    int a;
    cin>> a;
    on[a] = -1;
  }
  Segment<int> seg(on,0,[](int a,int b){
    if(a == -1 && b != -1) return b;
    if(a != -1 && b == -1) return a;
    if(a == -1 && b == -1) return -1;
    return a + b + 1;
  } , [](int node,int x){return x;});
  cout << max(0,seg.get_inter(0,n)) << endl;
}
