#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll,ll>;
#define fix(x) fixed << setprecision(x)
#define asc(x) x, vector<x>, greater<x>
#define rep(i, n) for(ll i = 0; i < n; ++i)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL << 62), MOD = 998244353;
constexpr int INF = (1 << 30);

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int l,n;
    cin >> l >> n;
    int z = 0;
    vector<bitset<100001>> bit(2,0);
    auto v = bit;
    bit[0][50000] = 1;
    rep(i,n+1){
        int x;
        if(i==n) x = l;
        else cin >> x;
        int y = x-z;
        auto upd = v;
        upd[0] |= bit[0] << y;
        if(i){
            upd[1] |= bit[1]>>y;
            upd[1] |= bit[0]>>y;
            upd[0] |= bit[1]<<y;
        }
        z = x;
        bit = upd;
    }
    int ans = INF;
    rep(i,100001){
        if(bit[1][i]){
            chmin(ans, (int)abs(50000-i));
        }
    }
    cout << ans << '\n';
    return 0;
}