#include <bits/stdc++.h>
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
const ll ILL=2167167167167167167;
const int INF=2100000000;
#define rep(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define all(p) p.begin(),p.end()
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,T b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,T b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
bool yneos(bool a,bool upp=0){if(a){cout<<(upp?"YES\n":"Yes\n");}else{cout<<(upp?"NO\n":"No\n");}return a;}
template<class T> void vec_out(vector<T> &p,int ty=0){
    if(ty==2){cout<<'{';for(int i=0;i<(int)p.size();i++){if(i){cout<<",";}cout<<'"'<<p[i]<<'"';}cout<<"}\n";}
    else{if(ty==1){cout<<p.size()<<"\n";}for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}}
template<class T> T vec_min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T vec_max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T vec_sum(vector<T> &a){T ans=T(0);for(auto &x:a) ans+=x;return ans;}
int pop_count(long long a){int res=0;while(a){res+=(a&1),a>>=1;}return res;}
template<class T> bool inside(T l,T x,T r){return l<=x&&x<r;}


void solve();
// CYAN / FREDERIC
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    rep(i, 0, t) solve();
}

void solve(){
    int N, K;
    cin >> N >> K;
    vector<int> A(K);
    rep(i, 0, K) cin >> A[i];
    rep(i, 0, K) A.push_back(A[i] + 2 * N);
    vector<int> p(N), nx(N);
    ll L = 0;
    rep(i, 0, K * 2){
        while (L != N && L * 2 < A[i]) nx[L] = A[i + 1] / 2 + 1, p[L++] = A[i] / 2 + 1;
    }
    int l = 0, r = N / K + 1;
    auto mul = [&](vector<int> a, vector<int> b) -> vector<int> {
        vector<int> c(a.size());
        rep(i, 0, a.size()) c[i] = a[b[i]];
        return c;
    };
    auto my_pow = [&](vector<int> f, int t) -> vector<int> {
        vector<int> res(f.size());
        rep(i, 0, f.size()) res[i] = i;
        while (t){
            if (t & 1) res = mul(f, res);
            t /= 2;
            if (t) f = mul(f, f);
        }
        return res;
    };
    while (r - l > 1){
        int m = (r + l) / 2;
        vector<int> q(N * 2 + 1);
        q[N * 2] = N * 2;
        rep(i, 0, N){
            q[i] = max(i + m, p[i]);
            if (q[i] >= nx[i]) q[i] = N * 2;
            q[i + N] = min(N * 2, q[i] + N);
        }
        //cout << m << "\n";
        //vec_out(q);
        q = my_pow(q, K);
        //vec_out(q);
        rep(i, 0, N) if (q[i] <= i + N) l = m;
        if (l != m) r = m;
    }
    //vec_out(nx);
    //vec_out(p);
    //vec_out(A);
    cout << l * 2 << "\n";
}