#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include "bits/stdc++.h"
#include <random>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);++i)
#define rep2(i,a,b) for(int i=(a);i<(b);++i)
#define rrep(i,n) for(int i=(n)-1;i>=0;--i)
#define rrep2(i,a,b) for(int i=(a)-1;i>=b;--i)
#define range(i,a,b,c) for(int i=a;\
                    c>0?i<b:\
                    i>b;\
                    i+=c)
#define chmax(a, b) (a = (a) < (b) ? (b) : (a))
#define chmin(a, b) (a = (a) > (b) ? (b) : (a))
using ll = long long;
using ull = unsigned long long;
using ld = long double;
#define all(a) begin(a),end(a)
#define ifnot(a) if(not (a))
#define dump(x)  cerr << #x << " = " << (x) << endl
#define int long long
#ifdef _MSC_VER
const bool test = true;
#else 
const bool test = false;
#endif

int dx[] = { 1,0,-1,0 };
int dy[] = { 0,1,0,-1 };
const int INF = 1 << 28;
const ll INFL = (ll)1 << 58;
ll mod = (int)1e9 + 7;
const double eps = 1e-10;
typedef long double Real;
// return -1, 0, 1
int sgn(const Real& r) { return (r > eps) - (r < -eps); }
int sgn(const Real& a, const Real &b) { return sgn(a - b); }

//.....................
const int MAX = (int)2e5 + 5;

vector<string> split(const string &str, char sep) {
	vector<string> v;
	stringstream ss(str);
	string buffer;
	while (getline(ss, buffer, sep)) {
		v.push_back(buffer);
	}
	return v;
}

template<class InputIterator>
int sum(InputIterator begin, InputIterator end) {
	return accumulate(begin, end, 0ll);
}

string reverse_str(string s) {
	reverse(all(s));
	return s;
}

int N, M;
int a[MAX];
bool c[MAX];

void solve() {
	cin >> N >> M;
	rep(i, M) {
		cin >> a[i];
		c[a[i]] = true;
	}

	int b = 1;
	int cnt = 0;
	rep(loop, 20) {
		if (b >= N) break;
		for (int i = 0; i + b < N; i += 2*b) {
			if (c[i]) c[i] = c[i + b];
			else if (c[i + b]) c[i] = c[i];
			else cnt++;
		}
		b *= 2;
	}
	cout << cnt << endl;
}

signed main() {
	srand(time(NULL));
	int T = (int)1e15;
	cout << fixed << setprecision(15);
	rep(i, T) {
		char s[MAX];
		if (scanf("%s", s) == EOF) break;
		int n = strlen(s);
		for (int i = n - 1; i > -1; i--) {
			ungetc(s[i], stdin);
		}
		solve();
	}
	return 0;
}