#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ull = unsigned long long;
template <class T> bool chmax(T &a, const T &b) {if (a < b) {a = b; return 1;} return 0;}
template <class T> bool chmin(T &a, const T &b) {if (b < a) {a = b; return 1;} return 0;}
const int INF = INT_MAX / 2;
const ll LINF = 1LL << 60;
const vector<int> dx = {-1, 0, 1, 0}, dy = {0, 1, 0, -1};
using mint = modint998244353;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int l, n;
  cin >> l >> n;
  vector<int> x(n);
  for (int i = 0; i < n; i++) {
    cin >> x[i];
  }
  map<int, int> mp;
  mp[x[0]] = 1;
  for (int i = 1; i < n; i++) {
    map<int, int> mp2;
    for (int j = -(int)1e5; j <= (int)1e5; j++) {
      if (mp.count(j)) {
        mp2[j + x[i] - x[i - 1]] = 1;
        mp2[j - (x[i] - x[i - 1])] = 1;
      }
    }
    swap(mp, mp2);
  }
  int ans = INF;
  for (int i = -(int)1e5; i <= (int)1e5; i++) {
    if (mp.count(i)) {
      chmin(ans, abs(i - (l - x[n - 1])));
    }
  }
  cout << ans << '\n';
  return 0;
}