#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <map>
using namespace std;

#define ll long long int
#define ti4 tuple<int,int,int,int>
#define pii pair<ll,ll>

int main()
{
    int N, M;
    cin >> N >> M;
    int d[20003];
    fill(d, d + 20003, 0);
    const int center = 10001;
    for(int i = 0;i < M;i++)
    {
        int p;
        cin >> p;
        d[p + center] = 1;
    }
    int b[20003];
    for(int i = 0;i < 20003;i++)
    {
        b[i] = b[i-1] + d[i];
    }
    int mindist = 10000000;
    int left = 0;
    int right = 0;
    int current = 0;
    while(left < 20002)
    {
        while(current < N && right < 20003)
        {
            right++;
            current = b[right] - b[left];
        }
        if(right == 20003)
        {
            break;
        }
        while(current == N && left < right)
        {
            int ldist = left + 1 - center;
            int rdist = right - center;
            if(ldist * rdist >= 0)
            {
                mindist = min(mindist, max(abs(ldist), (rdist)));
            }
            else
            {
                mindist = min(mindist, min(abs(ldist), (rdist)) + rdist - ldist);
            }
            if(mindist == 0)
            {
                //cout << left << "," << right;
            }
            left++;
            current = b[right] - b[left];
        }
        if(current == N)
        {
            right++;
        }
    }
    cout << mindist << endl;
    return 0;
}