#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N,K; cin >> N >> K;
    vector<int> A(K);
    for(auto &a : A) cin >> a;
    for(int i=0; i<K; i++) A.push_back(A.at(i)+2*N);

    int answer = 0;
    for(int i=0; i<=2*N/K+1; i+=2){
        int low = 1,high = N;
        while(high-low > 1){
            int mid = (high+low)/2;
            int now = 0,pos = i;
            while(now < K){
                int apos = upper_bound(A.begin(),A.end(),pos)-A.begin();
                int next = max(A.at(apos)+1,pos+mid*2);
                if(next > i+2*N) break;
                now++; pos = next;
            }
            if(now == K) low = mid;
            else high = mid;
        }
        answer = max(answer,low*2);
    }
    cout << answer << endl;
}