#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
#define rep(i, a, b) for(ll i = a; i < b; ++i)
#define rrep(i, a, b) for(ll i = a; i >= b; --i)
constexpr ll inf = 4e18;
struct SetupIO {
    SetupIO() {
        ios::sync_with_stdio(0);
        cin.tie(0);
        cout << fixed << setprecision(30);
    }
} setup_io;
int main(void) {
    int n, k;
    cin >> n >> k;
    n *= 2;
    vector<int> a(2 * k);
    rep(i, 0, k) {
        cin >> a[i];
        a[k + i] = a[i] + n;
    }
    int mi = 1e9, s = -1;
    rep(i, 0, 2 * k - 1) {
        if(a[i + 1] - a[i] < mi) {
            mi = a[i + 1] - a[i];
            s = i;
        }
    }
    int ans = 0;
    rep(i, a[s] + 1, a[s + 1]) {
        int lower = 1, upper = n / k + 5;
        while(upper - lower > 1) {
            int mid = (lower + upper) / 2;
            int cur = i, idx = s;
            bool res = true;
            rep(j, 0, k - 1) {
                cur += mid;
                if(cur >= a[idx + 2]) {
                    res = false;
                    break;
                }
                cur = max(cur, a[idx + 1] + 1);
                idx++;
            }
            if(i + n - cur < mid) res = false;
            if(res) {
                lower = mid;
            } else {
                upper = mid;
            }
        }
        ans = max(ans, lower);
    }
    if(ans % 2 == 1) ans--;
    cout << ans << '\n';
}