#pragma GCC optimize("O2")
#include <algorithm>
#include <array>
#include <bit>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfenv>
#include <cfloat>
#include <chrono>
#include <cinttypes>
#include <climits>
#include <cmath>
#include <compare>
#include <complex>
#include <concepts>
#include <cstdarg>
#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <fstream>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <ios>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <new>
#include <numbers>
#include <numeric>
#include <ostream>
#include <queue>
#include <random>
#include <ranges>
#include <set>
#include <span>
#include <sstream>
#include <stack>
#include <streambuf>
#include <string>
#include <tuple>
#include <type_traits>
#include <variant>

#define int ll
#define INT128_MAX (__int128)(((unsigned __int128) 1 << ((sizeof(__int128) * __CHAR_BIT__) - 1)) - 1)
#define INT128_MIN (-INT128_MAX - 1)

namespace R = std::ranges;
namespace V = std::views;

using namespace std;

using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<long long, long long>;
using tiii = tuple<int, int, int>;
using ldb = long double;
//#define double ldb

template<class T>
ostream& operator<<(ostream& os, const pair<T, T> pr) {
  return os << pr.first << ' ' << pr.second;
}
template<class T, size_t N>
ostream& operator<<(ostream& os, const array<T, N> &arr) {
  for(const T &X : arr)
    os << X << ' ';
  return os;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T> &vec) {
  for(const T &X : vec)
    os << X << ' ';
  return os;
}


signed main() {
  ios::sync_with_stdio(false), cin.tie(NULL);

  int l, n; cin >> l >> n;
  vector<int> x(n + 2);
  for(int i = 1; i <= n; i++)
    cin >> x[i];
  x[n + 1] = l;

  vector dp(2 * l + 1, vector<bool>(2, false));
  dp[x[1] + l][0] = true;
  for(int i = 2; i <= n + 1; i++) {
    vector tmp(2 * l + 1, vector<bool>(2, false));
    for(int j = -l; j <= l; j++)
      for(int k : {0, 1})
        if (dp[j + l][k])
          tmp[j + l + x[i] - x[i - 1]][k] = tmp[x[i] - x[i - 1] - j + l][1 - k] = true;
    dp.swap(tmp);
  }

  int ans = INT_MAX;
  for(int j = -l; j <= l; j++)
    if (abs(j) < ans and dp[j + l][1])
      ans = abs(j);

  cout << ans << '\n';

  return 0;
}