#include<bits/stdc++.h>
#define mp make_pair
#define ll long long
#define il inline
using namespace std;
const int maxn=10000010;
il ll read(){
	ll x=0;
	char c=getchar();
	for(;!(c>='0'&&c<='9');c=getchar());
	for(;c>='0'&&c<='9';c=getchar())
		x=(x<<1)+(x<<3)+c-'0';
	return x;
}
ll n,m,ans;
map<ll,int>a;
ll st[maxn];
void calc(ll l,ll r){
	if(l==r){
		ans+=(a[l]==-1);
		return ;
	}
	ll mid=l+r>>1,top=0;
	for(auto it=a.lower_bound(l);it!=a.end();it++) 
		if(it->first>r) break;
		else st[++top]=it->first;
	for(int i=1;i<=top;i++){
		ll x=st[i],y=mid*2-x+1;
		ll w1=a[x],w2=a[y];
		if(w1==-1&&w2==-1) ans++;
		else if(w1==-1) a[x]=w2,ans++;
		else if(w2==-1) a[y]=w1,ans++;
		else if(w1!=w2) a[x]=a[y]=-1;
	}
	calc(l,mid);
}
int main(){
	n=read(),m=read();
	for(int i=1;i<=m;i++) a[read()]=1;
	calc(1,n);
	printf("%lld\n",ans);
	return 0;
}