// 想定TLE解法
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
    ll n,k; cin >> n >> k;
    vector<ll> a(k+1);
    for (size_t i=0; i<k; i++) cin >> a[i];
    a[k] = a[0] + 2*n;

    vector<ll> an = a;
    for (size_t i=0; i<=k; i++) an[i] -= a[0];
    ll ans = 0;
    for (ll s = an[0]+1; s < an[1]; s++) {
        ll ok = 0, ng = n;
        auto is_ok = [&] (ll mid) -> bool {
            ll x = 2 * mid;
            ll t = s;
            for (size_t i=0; i<k-1; i++) {
                if (t + x >= an[i+2]) return false;
                t = max(t+x, an[i+1]+1);
            }
            return t+x <= s+2*n;
        };
        while (ng - ok > 1) {
            ll mid = (ok + ng) / 2;
            if (is_ok(mid)) ok = mid;
            else ng = mid;
        }
        ans = max(ans, 2*ok);
    }
    cout << ans << endl;
}