#include<iostream>
#include<algorithm>
using namespace std;

int main()
{
	int n, m, d[1001], i, j;

	cin >> n >> m;

	for (i = 1; i <= m; i++) cin >> d[i];

	sort(d + 1, d + m + 1);

	int count = 0, min = 0, max = 0;
	for (i = 0; i <= 10000; i++)
	{
		for (j = 1; j <= m; j++)
		{
			if (d[j] == -i)
			{
				min = -i;
				count++;
				if (count == n) goto END;
			}
			else if (d[j] == i)
			{
				max = i;
				count++;
				if (count == n) goto END;
				break;
			}
		}
	}

END: 
	if (max == 0) cout << -min << endl;
	else if (min == 0) cout << max << endl;
	else cout << max - min * 2 << endl;
}