#include <bits/stdc++.h>
using namespace std;

int main()
{
    int L, N;
    cin >> L >> N;
    vector<int> X(N + 2);
    X[0] = 0;
    X[N + 1] = L;
    for (int i = 0; i < N; i++)
    {
        int x;
        cin >> x;
        X[i + 1] = x;
    }
    vector<int> A;
    for (int i = 0; i < N + 1; i++)
    {
        A.push_back(X[i + 1] - X[i]);
    }
    N = A.size();
    int M = 2 * L + 10;
    vector<vector<vector<int>>> dp(N + 1, vector<vector<int>>(M, vector<int>(2)));
    dp[0][L][0] = 1;
    for (int i = 0; i < N; i++)
    {
        int a = A[i];
        for (int j = 0; j < M; j++)
        {
            for (int k = 0; k < 2; k++)
            {
                if (dp[i][j][k] == 0)
                {
                    continue;
                }
                if (k == 0)
                {
                    dp[i + 1][j + a][k] = 1;
                    if (i > 0)
                    {
                        dp[i + 1][j - a][1 - k] = 1;
                    }
                }
                else
                {
                    dp[i + 1][j - a][k] = 1;
                    if (i > 0)
                    {
                        dp[i + 1][j + a][1 - k] = 1;
                    }
                }
            }
        }
    }
    int ans = pow(10, 9);
    for (int j = 0; j < M; j++)
    {
        if (dp[N][j][1] == 1)
        {
            ans = min(ans, abs(j - L));
        }
    }
    cout << ans << endl;
}