#pragma GCC optimize(2,3,"Ofast","inline")
#include <bits/stdc++.h>
#define int long long
#define double long double
#define lowbit(i) (i&(-i))
#define mid ((l+r)>>1)
using namespace std;
vector<pair<int,int>> vc,tmp;
map<int,int> mp,tag;
int n,ans;
void operate(int pos){
	int sta1=mp[pos],sta2=mp[n+1-pos];
	if(sta1==0&&sta2==0){
		tmp.push_back(make_pair(pos,0));
	}
	if(sta1==1&&sta2==0){
		tmp.push_back(make_pair(pos,2));
		ans++;
	}
	if(sta1==2&&sta2==0){
		tmp.push_back(make_pair(pos,0));
	}
	if(sta1==0&&sta2==1){
		tmp.push_back(make_pair(pos,2));
		ans++;
	}
	if(sta1==1&&sta2==1){
		tmp.push_back(make_pair(pos,1));
	}
	if(sta1==2&&sta2==1){
		tmp.push_back(make_pair(pos,1));
	}
	if(sta1==0&&sta2==2){
		tmp.push_back(make_pair(pos,0));
	}
	if(sta1==1&&sta2==2){
		tmp.push_back(make_pair(pos,1));
	}
	if(sta1==2&&sta2==2){
		tmp.push_back(make_pair(pos,2));
	}
}
signed main(){
	int m; cin>>n>>m;
	for(int i=1;i<=m;i++){
		int a; cin>>a;
		vc.push_back(make_pair(a,1)); 
	}
	while(n!=1){
		mp.clear(),tag.clear();
		for(auto v:vc) mp[v.first]=v.second,tag[v.first]=1;
		tmp.clear();
		for(auto v:vc){
			if(v.first<=n/2){
				operate(v.first);
			}
			else if(!tag[n+1-v.first]){
				operate(n+1-v.first);
			}
		}
		sort(tmp.begin(),tmp.end());
		vc=tmp;
		n/=2;
	}
	cout<<ans;
	return 0;
}