#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;
  int pos = 0;
  for (int i = 0; i < n; i++) {
    int val;
    cin >> val;
    x.push_back(val - pos);
    pos = val;
  }
  x.push_back(l - pos);
  bitset<300010> dp, dp2;
  const int base = 150000;
  dp[base + x[0]] = 1;
  for (int i = 1; i < n + 1; i++) {
    bitset<300010> nxt, nxt2;
    nxt |= (dp << x[i]);
    nxt |= (dp2 << x[i]);
    nxt2 |= (dp >> x[i]);
    nxt2 |= (dp2 >> x[i]);
    swap(dp, nxt);
    swap(dp2, nxt2);
  }
  int ans = INF;
  for (int i = 0; i <= 300009; i++) {
    if (!dp2[i]) {
      continue;
    }
    chmin(ans, abs(base - i));
  }
  cout << ans << '\n';
  return 0;
}