//http://yukicoder.me/problems/780
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <functional>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <limits>
#include <sstream>
#include <typeinfo>

using namespace std;

int main() {
	int N, M;
	cin >> N >> M;
	vector<int> D(M);
	for (int i = 0; i < M; i++) {
		cin >> D[i];
	}
	sort(D.begin(), D.end());
	int start = 1000000000;
	int cur = 0;
	for (int i = 0; i < M; i++) {
		if (start > abs(D[i])) {
			start = abs(D[i]);
			cur = i;
		}
	}
	typedef std::tuple<int, int, int, int, int> t; //(-ans, cur, left, right, rest)
	priority_queue<t> q;

	int ans = abs(D[cur]);
	q.push(make_tuple(-ans, cur, cur, cur, N - 1));
	while (!q.empty()) {
		int rev_ans, left, right, rest;
		tie(rev_ans, cur, left, right, rest) = q.top(); q.pop();
		ans = -rev_ans;
		if (rest == 0)
			break;
		if (left-1 >= 0) {
			int d = abs(D[left - 1] - D[cur]);
			q.push(make_tuple(rev_ans - d, left - 1, left - 1, right, rest - 1));
		}
		if (right + 1 < M) {
			int d = abs(D[right + 1] - D[cur]);
			q.push(make_tuple(rev_ans - d, right + 1, left, right + 1, rest - 1));
		}
	}
	cout << ans << endl;
	return 0;
}