// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
#include <unordered_map>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define TEST cerr << "TEST" << endl
#define AMARI 998244353
// #define AMARI 1000000007
#define TIME_LIMIT 1980000
#define el '\n'
#define El '\n'

#define MULTI_TEST_CASE false
void solve(void) {
    int l,n;
    cin >> l >> n;
    vector<int> x(n);
    vector<bool> kireme(l,false);
    for(int i = 0; i < n; i++){
        cin >> x[i];
    }
    x.push_back(l);
    n++;
    //dp0[i] := 赤側を選んでる時の、i番目の切れ目まで見た時のありえるA-Bの値の集合
    vector<set<int>> dp0(n + 1),dp1(n + 1);
    dp0[0].insert(0);
    for(int i = 0; i < n; i++){
        int dif = x[i] - (i == 0 ? 0 : x[i - 1]);
        for(set<int>::iterator it = dp0[i].begin(); it != dp0[i].end(); it++){
            dp0[i + 1].insert(*it + dif);
            if(i != 0)dp1[i + 1].insert(*it - dif);
        }
        for(set<int>::iterator it = dp1[i].begin(); it != dp1[i].end(); it++){
            dp0[i + 1].insert(*it + dif);
            dp1[i + 1].insert(*it - dif);
        }
    }
    int ans = INT_MAX;
    for(set<int>::iterator it = dp1[n].begin(); it != dp1[n].end(); it++){
        ans = min(ans,abs(*it));
    }
    cout << ans << el;
    return;
}

void calc(void) {
    return;
}

signed main(void) {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    calc();
    int t = 1;
    if(MULTI_TEST_CASE) cin >> t;
    while(t--) {
        solve();
    }
    return 0;
}