#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=100009;
set<LL> st;
map<LL,int> mp0,mp1;
LL n,m,a[N];
int main()
{
	//freopen("1.txt","r",stdin);
	scanf("%lld%lld",&n,&m);
	for(int i=1;i<=m;i++) 
		scanf("%lld",&a[i]),mp0[a[i]]=1,mp1[a[i]]=0,st.insert(min(n-a[i]+1,a[i]));
	while(n>1)
	{
		for(int i:st) 
		{
			if(!mp1.count(i)) mp1[i]=1;
			if(!mp1.count(n-i+1)) mp1[n-i+1]=1;
			int tmp1,tmp2;
			tmp1=min({mp0[i]+mp0[n-i+1],mp0[i]+mp1[n-i+1]+1,mp1[i]+mp0[n-i+1]+1});
			tmp2=min({mp1[i]+mp1[n-i+1],mp0[i]+mp1[n-i+1]+1,mp1[i]+mp0[n-i+1]+1});
			mp0[i]=tmp1;
			mp1[i]=tmp2;
			mp0.erase(n-i+1);
			mp1.erase(n-i+1);
		}
		m=0;
		for(auto i:st) a[++m]=min(i,n-i+1);
		n/=2;
		st.clear();
		for(int i=1;i<=m;i++) st.insert(a[i]);
	}
	printf("%d",min(mp0[1],mp1[1]));
}

/*
8 3
1 5 8

 */