#include <vector>
#include <string>
#include <algorithm>
#include <numeric>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <utility>
#include <list>
#include <map>
#include <queue>
#include <iterator>
using namespace std;

using ull = unsigned long long;
using ll = long long;

int main()
{
    int n,m;
    cin >> n >> m;

    vector<int> data(20002);
    for(int i = 0; i < m; ++i)
    {
        int n;
        cin >> n;
        data.at(10000 + n) = 1;
    }


    int minimum = 1000000;
    for(int j = -10000; j < 10000; ++j)
    {
        int count = 0;
        int s = abs(j)*2;
        bool check = false;
        if(data.at(10000 - j) != 1) continue;
        for(int i = 10000 - j; i < data.size(); ++i)
        {
            if(data.at(i) == 1) ++count;
            if(count == n)
            {
                check = true;
                break;
            }
            ++s;
        }
        if(check)
            minimum = min(minimum,s);
        check = false;
    }

    cout << minimum-1<< endl;
}