#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(int i=0; i<int(n); i++)
const i64 INF = 1001001001001001001;
template<typename A> void chmin(A& l, const A& r){ if(r < l) l = r; }
template<typename A> void chmax(A& l, const A& r){ if(l < r) l = r; }
#include <atcoder/modint>
using Modint = atcoder::static_modint<998244353>;
using namespace std;

void testcase(){
    i64 N, K; cin >> N >> K;
    vector<i64> A(K);
    rep(i,K) cin >> A[i];
    rep(i,K+1) A.push_back(A[i] + N*2);
    for(i64 k=N/K*2; k>=2; k-=2){
        vector<i64> lf = A;
        rep(i,K*2-1) chmax(lf[i+1], lf[i] + k);
        i64 ok = true;
        for(i64 i=K; i<2*K; i++) if(lf[i-1] + k + 1 > A[i+1] - 1) ok = false;
        if(ok){ cout << k << endl; return; }
    }
}

int main(){
    ios::sync_with_stdio(false); cin.tie(nullptr);
    rep(t,3) testcase();
    return 0;
}