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

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

#define rep(i,n) for(int i = 0; i < (int)n; i++)
#define FOR(n) for(int i = 0; i < (int)n; i++)
#define repi(i,a,b) for(int i = (int)a; i < (int)b; i++)
#define all(x) x.begin(),x.end()
//#define mp make_pair
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
#define pii pair<int,int>
#define vpii vector<pair<int,int>>

template<typename T>
bool chmax(T &a, const T b) {if(a<b) {a=b; return true;} else {return false;}}
template<typename T>
bool chmin(T &a, const T b) {if(a>b) {a=b; return true;} else {return false;}}

using ll = long long;
using ld = long double;
using ull = unsigned long long;

const ll INF = numeric_limits<long long>::max() / 2;
const ld pi = 3.1415926535897932384626433832795028;
const ll mod = 998244353;
int dx[] = {1, 0, -1, 0, -1, -1, 1, 1};
int dy[] = {0, 1, 0, -1, -1, 1, -1, 1};

#define int long long

void solve() {
    int n, k;
    cin >> n >> k;
    vector<bool> strawberry(2*n, false);
    FOR(k) {
        int a;
        cin >> a;
        strawberry[a] = true;
    }

    auto cal_dist = [n](int from, int to) -> int {
        if(from < to) return to - from;
        return 2*n - from + to;
    };

    vi bits;
    FOR(20) if(k>>i&1) bits.emplace_back(i);

    int l = 0, r = 2*n/k+1;
    while(r-l > 1) {
        int mid = (l + r) / 2;
        vi to(2*n, -1);
        int cur_r = 0;
        int strawberry_cnt = 0;
        for(int i = 0; i < 2*n; i += 2) {
            while(strawberry_cnt == 0 || cur_r-i < mid) {
                if(strawberry[(cur_r+1) % (2*n)]) ++strawberry_cnt;
                cur_r += 2;
            }
            to[i] = cur_r % (2 * n);
            if(strawberry[(i + 1) % (2*n)]) --strawberry_cnt;
        }

        vvi distd(20, vi(2*n, -1)), tod(20, vi(2*n, -1));
        for(int i = 0; i < 2*n; i += 2) distd[0][i] = cal_dist(i, to[i]);
        tod[0] = to;
        rep(lg, 19) for(int i = 0; i < 2*n; i += 2) {
            tod[lg+1][i] = tod[lg][tod[lg][i]];
            distd[lg+1][i] = distd[lg][i] + distd[lg][tod[lg][i]];
        }

        // cerr << mid << endl;
        // FOR(2*n) cout << to[i] << " \n"[i == 2*n-1];

        bool can_cut = false;

        for(int i = 0; i < 2*n; i += 2) {
            int cur_place = i, dist = 0;
            for(int bit : bits) {
                dist += distd[bit][cur_place];
                cur_place = tod[bit][cur_place];
            }
            if(dist <= 2 * n) {
                // cerr << i << " " << dist << endl;
                can_cut = true;
            }
        }

        if(can_cut) l = mid;
        else r = mid;
    }

    cout << l << endl;
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}