#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;

template<typename T>
vector<vector<T>> vec2d(int n, int m, T v){
    return vector<vector<T>>(n, vector<T>(m, v));
}

template<typename T>
vector<vector<vector<T>>> vec3d(int n, int m, int k, T v){
    return vector<vector<vector<T>>>(n, vector<vector<T>>(m, vector<T>(k, v)));
}

template<typename T>
void print_vector(vector<T> v, char delimiter=' '){
    if(v.empty()) {
        cout << endl;
        return;
    }
    for(int i = 0; i+1 < v.size(); i++) cout << v[i] << delimiter;
    cout << v.back() << endl;
}

const int X = 100000;
bool ok[105][2][200005];

void set_ok(int i, int j, int k, bool f){
    ok[i][j][k+X] = f;
}

bool get_ok(int i, int j, int k){
    return ok[i][j][k+X];
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    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;
    set_ok(0, 0, 0, true);
    for(int i = 1; i <= n+1; i++){
        int len = x[i]-x[i-1];
        for(int j = 0; j < 2; j++){
            for(int diff = -l; diff <= l; diff++){
                if(!get_ok(i-1, j, diff)) continue;

                int diff_next = [&](){
                    if(j == 0) return diff+len;
                    return diff-len;
                }();
                if(i != 1){
                    set_ok(i, j, diff_next, true);
                }
                diff_next = [&](){
                    if(j == 0) return diff-len;
                    return diff+len;
                }();
                set_ok(i, j^1, diff_next, true);
            }
        }
    }
    int ans = l;
    for(int diff = -l; diff <= l; diff++){
        if(get_ok(n+1, 0, diff)) {
            chmin(ans, abs(diff));
        }
    }
    cout << ans << endl;
}