#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001LL
vector<long long> merge(vector<long long> &a,vector<long long> &b){
	vector<long long> ret(a.size());
	rep(i,a.size()){
		int to = i + a[i];
		to %= a.size();
		ret[i] = a[i] + b[to];
		
	}
	return ret;
}
int main(){
	
	long long N,K;
	cin>>N>>K;
	N*=2;
	N/=2;
	vector<long long> a(N);
	rep(i,K){
		int t;
		cin>>t;
		t--;
		a[t/2]++;
	}
	rep(i,N)a.push_back(a[i]);
	vector<int> nxt(N);
	{
		int last = -1;
		for(int i=N*2-1;i>=0;i--){
			if(a[i])last = i;
			if(last!=-1)nxt[i%N] = last-i+1;
		}
	}
	a.insert(a.begin(),0);
	rep(i,a.size()-1){
		a[i+1]+=a[i];
	}
	int ok = 1,ng = N;
	while(ng-ok>1){
		int mid = (ok+ng)/2;
		
		vector<long long> dp(N);
		rep(i,N){
			int t = a[i+mid] - a[i];
			
			dp[i] = max(nxt[i],mid);
			
		}

		vector<long long> cur(N,0);
		rep(i,19){
			if((K>>i)&1)cur = merge(cur,dp);
			dp= merge(dp,dp);
		}
		long long mini = Inf64;
		rep(i,N){
			if(cur[i]!=-1){
				mini = min(mini,cur[i]);
			}
		}
		
		if(mini<=N)ok = mid;
		else ng = mid;
	}
	cout<<ok*2<<endl;
	return 0;
	
}