/* -*- coding: utf-8 -*-
 *
 * 2542.cc:  No.2542 Yokan for Two - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_L = 100000;
const int MAX_L2 = MAX_L * 2;
const int MAX_N = 100;
const int INF = 1 << 30;

/* typedef */

/* global variables */

int xs[MAX_N];
bool dp[MAX_N][MAX_L2 + 1];

/* subroutines */

/* main */

int main() {
  int l, n;
  scanf("%d%d", &l, &n);
  for (int i = 0; i < n; i++) scanf("%d", xs + i);

  int j0 = l, j1 = l;
  dp[0][j0] = true;
  for (int i = 0; i + 1 < n; i++) {
    int dx = xs[i + 1] - xs[i];
    for (int j = j0; j <= j1; j++)
      if (dp[i][j])
	dp[i + 1][j + dx] = dp[i + 1][j - dx] = true;
    j0 -= dx, j1 += dx;
  }

  int mind = INF, d0 = xs[0] - (l - xs[n - 1]);
  for (int j = j0; j <= j1; j++)
    if (dp[n - 1][j])
      mind = min(mind, abs(j - l + xs[0] - (l - xs[n - 1])));

  printf("%d\n", mind);
  return 0;
}