#include <bits/stdc++.h>

using ll = std::int64_t;

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    ll N, K;
    std::cin >> N >> K;

    std::vector<int> A(2 * K);
    for(int i=0;i<K;i++){
        std::cin >> A[i];
        A[i + K] = A[i] + 2 * N;
    }

    auto mul = [](std::vector<int> a, std::vector<int> b){
        std::vector<int> res(a.size());
        for(int i=0;i<a.size();i++){
            res[i] = b[a[i]];
        }
        return res;
    };

    auto pow = [&mul](std::vector<int> a, int n){
        std::vector<int> res(a.size());
        std::iota(res.begin(), res.end(), 0);       
        while(n > 0){
            if(n & 1){res = mul(res, a);}
            a = mul(a, a);
            n /= 2;
        }
        return res;
    };

    auto f = [&](int x){
        std::vector<int> B(2 * N + 1);
        B[2 * N] = 2 * N;
        for(int i=0;i<2*N;i++){
            int j = std::lower_bound(A.begin(), A.end(), 2 * i) - A.begin();
            if(j == A.size()){
                B[i] = 2 * N;
            }else{
                B[i] = std::min<int>(std::max((A[j] + 1) / 2, i + x), 2 * N);
            }
        }

        std::vector<int> C = pow(B, K);

        for(int i=0;i<N;i++){
            if(C[i] <= i + N){
                return true;
            }
        }
        return false;
    };

    int ok = 1, ng = N + 1;
    while(ng - ok > 1){
        int m = (ok + ng) / 2;
        if(f(m)){
            ok = m;
        }else{
            ng = m;
        }
    }

    int res = ok * 2;
    std::cout << res << std::endl;
}