#include<algorithm>
#include<vector>
#include<cstdio>
#include<map>
#include<cstring>
#include<iostream>
#include<cstdlib>
#include<queue>
#include<iomanip>
#include<stdlib.h>
#include<set>
#include<cassert>
#include<cmath>
#define int long long
#define y0 ahfweaipuhrfil
#define y1 sdafjhiuhfalwu
#define x0 adlkhfguywelsj
#define x1 guyoago2o8iraj
#define lowbit(i) (i&-i)
using namespace std;
const int N=3e3+10,inf=0x7fffffff,mod=1e9+7;
int n,m;
int ans=0;
set<int> s;
map<int,int> f[2];
signed main(){
    cin>>n>>m;
    for(int i=1;i<=m;i++){
        int a;
        cin>>a;
        s.insert(a);
        f[1][a]=0;f[0][a]=1;
    }
    int kkk=0;
    while(n>2){
        set<int> s1;
        for(auto i:s){
            int k=min(i,n-i+1);
            if(s.count(n-i+1)){
                f[1][k]=f[1][i]+f[1][n-i+1];
                f[0][k]=f[0][i]+f[0][n-i+1];
                s1.insert(k);
            }
            else{
                f[1][k]=f[1][i]+(1<<kkk);
                f[0][k]=f[0][i]+f[0][n-i+1];
                s1.insert(k);
            }
        }
        swap(s,s1);
        n>>=1;
        kkk<<=1;
    }
    cout<<min(f[1][1],f[0][1])<<endl;
    return 0;
}
/*
 
 8 3
 1 5 8
 */