#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define vi vector<int>
#define vl vector<ll>
#define vii vector<pair<int,int>>
#define vll vector<pair<ll,ll>>
#define vvi vector<vector<int>>
#define vvl vector<vector<ll>>
#define vvii vector<vector<pair<int,int>>>
#define vvll vector<vector<pair<ll,ll>>>
#define vst vector<string>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define mkunique(x) sort(all(x));(x).erase(unique(all(x)),(x).end())
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=1000005,INF=15<<26;

#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")


ll db[23][MAX];

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    ll N,K;cin>>N>>K;
    vl NG(2*N+3);
    for(int i=0;i<K;i++){
        int x;cin>>x;x--;
        NG[x]=true;
    }
    
    ll le=0,ri=N/K+3;
    while(ri-le>1){
        ll mid=(le+ri)/2;
        for(int t=0;t<22;t++) for(int i=0;i<2*N;i+=2) db[t][i]=-1;
        int j=1;
        int cn=0;
        for(int i=1;i<2*N;i+=2){
            while(j-i<2*mid||cn==0){
                if(NG[(j+1)%(2*N)]) cn++;
                j+=2;
            }
            db[0][i]=j-i;
            //cout<<mid<<" "<<j-i<<endl;
            if(NG[i+1]) cn--;
        }
        
        for(int t=1;t<=20;t++){
            for(int i=1;i<2*N;i+=2){
                if(db[t-1][i]>2*N){
                    db[t][i]=2*N+2;
                    continue;
                }
                int y=(i+db[t-1][i]);
                while(y>=2*N) y-=2*N;
                db[t][i]=db[t-1][i]+db[t-1][y];
            }
        }
        
        bool ok=false;
        
        for(int i=1;i<2*N;i+=2){
            ll now=i,sum=0;
            for(int t=20;t>=0;t--){
                if(K&(1<<t)){
                    sum+=db[t][now];
                    now=(now+db[t][now]);
                    while(now>=2*N) now-=2*N;
                }
                
                if(sum>2*N) break;
            }
            
            if(sum<=2*N) ok=true;
        }
        
        if(ok) le=mid;
        else ri=mid;
    }
    
    cout<<2*le<<endl;
}