#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <algorithm>
#include <atcoder/all>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define mod3 100000007
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvb vector<vector<bool>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int, int>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int, int>>
#define vpll vector<pair<ll, ll>>
#define vvpii vector<vector<pair<int, int>>>
#define vvpll vector<vector<pair<ll, ll>>>

// using mint = modint998244353;

template <typename T>
void debug(T e) {
    cerr << e << endl;
}

template <typename T>
void debug(vector<T> &v) {
    rep(i, v.size()) { cerr << v[i] << " "; }
    cerr << endl;
}

template <typename T>
void debug(vector<vector<T>> &v) {
    rep(i, v.size()) {
        rep(j, v[i].size()) { cerr << v[i][j] << " "; }
        cerr << endl;
    }
}

template <typename T>
void debug(vector<pair<T, T>> &v) {
    rep(i, v.size()) { cerr << v[i].first << " " << v[i].second << endl; }
}

template <typename T>
void debug(set<T> &st) {
    for (auto itr = st.begin(); itr != st.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(multiset<T> &ms) {
    for (auto itr = ms.begin(); itr != ms.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(map<T, T> &mp) {
    for (auto itr = mp.begin(); itr != mp.end(); itr++) {
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << H << " ";
    debug_out(T...);
}

int main() {
    ll N, K;
    cin >> N >> K;
    vector<ll> A(K);
    for (ll i = 0; i < K; i++) {
        cin >> A[i];
    }
    vector<ll> B(2 * K + 1);
    for (ll i = 0; i < K; i++) {
        B[i] = A[i];
        B[i + K] = A[i] + 2 * N;
    }
    B[2 * K] = A[0] + 4 * N;
    vector<ll> next(4 * N + 1, -1);
    for (ll i = 0; i < B[0]; i++) {
        next[i] = B[0];
    }
    for (ll i = 0; i <= 2 * K - 1; i++) {
        for (ll j = B[i]; j < min(4 * N + 1, B[i + 1]); j++) {
            next[j] = B[i + 1];
        }
    }
    // debug(next);
    ll min_val = 2 * N + 1;
    ll min_idx = -1;
    for (ll i = 0; i < K; i++) {
        if (min_val > B[i + 1] - B[i]) {
            min_val = B[i + 1] - B[i];
            min_idx = i;
        }
    }

    ll ok = 0;
    ll ng = 2 * N;
    while (ng - ok > 1) {
        ll mid = (ng + ok) / 2;

        ll mid2 = mid * 2;
        bool pos = false;

        for (ll i = B[min_idx] + 1; i < B[min_idx + 1]; i++) {
            ll now = i;
            bool possub = true;

            for (ll j = 0; j < K - 1; j++) {
                if (B[min_idx + j + 2] - now < mid2) {
                    possub = false;
                    break;
                }
                if (B[min_idx + j + 1] - now > mid2) {
                    now = B[min_idx + j + 1] + 1;
                } else {
                    now += mid2;
                }
                // debug_out(i, j, now);
            }
            if (i + 2 * N - now < mid2) {
                possub = false;
            }
            if (possub) {
                pos = true;
            }
        }
        if (pos) {
            ok = mid;
        } else {
            ng = mid;
        }
    }
    cout << ok * 2 << endl;
}