/* -*- coding: utf-8 -*-
 *
 * 2890.cc:  No.2890 Chiffon - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_K = 500000;
const int MAX_K2 = MAX_K * 2;

/* typedef */

/* global variables */

int as[MAX_K2 + 1];

/* subroutines */

bool check(int n, int k, int mini, int w) {
  for (int st = as[mini] + 1; st <= as[mini + 1]; st++) {
    bool ok = true;
    int x = st;
    for (int i = mini + 1; ok && i <= mini + k; i++) {
      x = max(x + w, as[i] + 1);
      ok = (x <= as[i + 1]);
    }
    if (ok && x <= st + n) return true;
  }
  return false;
}

/* main */

int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  int k2 = k * 2;
  for (int i = 0; i < k; i++) {
    scanf("%d", as + i);
    as[i] /= 2;
    as[k + i] = as[i] + n;
  }
  as[k2] = as[k] + n;

  int mind = as[1] - as[0], mini = 0;
  for (int i = 1; i < k; i++)
    if (mind > as[i + 1] - as[i])
      mind = as[i + 1] - as[i], mini = i;
  //printf(" mind=%d, mini=%d\n", mind, mini);

  int w0 = 1, w1 = n;
  while (w0 + 1 < w1) {
    int w = (w0 + w1) / 2;
    if (check(n, k, mini, w)) w0 = w;
    else w1 = w;
  }

  printf("%d\n", w0 * 2);
  
  return 0;
}