using System;
using System.Collections.Generic;
using static Assistant.Input;
using static Assistant.Debug;
using System.Linq;
using Assistant;

namespace ABC117C
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = ReadInt;
            var M = ReadInt;
            var X = ReadInts;

            if (N >= M)
            {
                Console.WriteLine("0");
                return;
            }

            X = X.OrderBy(x => x).ToArray();
            memo[] memos = new memo[M - 1];
            for (int i = 0; i < M - 1; i++)
            {
                memos[i] = new memo() { dist = X[i + 1] - X[i], index = i };
            }
            memos = memos.OrderBy(x => x.dist).ToArray();

            UnionFind uf = new UnionFind(M);
            for (int i = 0; i < M - N; i++)
            {
                int index = memos[i].index;
                uf.Unite(index, index + 1);
            }

            int[] parent = new int[M];
            for (int i = 0; i < M; i++)
            {
                parent[i] = uf.Find(i);
            }

            //Xとparent
            int ans = 0;
            int lastStart = 0;
            for (int i = 1; i <= M; i++)
            {
                if (i < M && parent[i] == parent[i - 1]) continue;
                else
                {
                    ans += X[i - 1] - X[lastStart];
                    lastStart = i;
                }
            }
            Console.WriteLine(ans);
        }

        struct memo
        {
            public int dist;
            public int index;
        }
    }


    class UnionFind
    {
        int[] par;
        int[] rank;
        int[] count;

        public UnionFind(int n)
        {
            Init(n);
        }

        void Init(int n)
        {
            par = new int[n];
            rank = new int[n];
            count = new int[n].Select(x => 1).ToArray();

            for (int i = 0; i < n; i++)
            {
                par[i] = i;
            }
        }

        public int Find(int x)
        {
            if (par[x] == x)
            {
                return x;
            }
            else
            {
                return par[x] = Find(par[x]);
            }
        }

        public bool Unite(int x, int y)
        {
            x = Find(x);
            y = Find(y);
            if (x == y) return false; //すでに要素を追加済みの場合、falseを返す。

            if (rank[x] < rank[y])
            {
                par[x] = y;
                count[y] += count[x];
            }
            else
            {
                par[y] = x;
                count[x] += count[y];
                if (rank[x] == rank[y]) rank[x]++;
            }
            return true;
        }
        public bool Same(int x, int y)
        {
            return Find(x) == Find(y);
        }

        public int Size(int x)
        {
            return count[Find(x)];
        }
    }

}

namespace Assistant
{
    static class Input
    {
        static List<string> line = new List<string>();
        static int index = 0;
        static String ReadNext()
        {
            if (line.Count <= index) line.AddRange(Console.ReadLine().Split());
            return line[index++];
        }
        public static int ReadInt => int.Parse(ReadNext());
        public static long ReadLong => long.Parse(ReadNext());
        public static int[] ReadInts => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static long[] ReadLongs => Console.ReadLine().Split().Select(long.Parse).ToArray();
    }

    static class Debug
    {
        static public void Draw2D(int[,] map, int mode = 1)
        {
#if DEBUG
            int W = map.GetLength(0);
            int H = map.GetLength(1);

            int[,] map2 = new int[W + 1, H + 1];
            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (i == 0 && j == 0) map2[i, j] = 0;
                    else if (i == 0) map2[i, j] = j - 1;
                    else if (j == 0) map2[i, j] = i - 1;
                    else map2[i, j] = map[i - 1, j - 1];
                }
            }

            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (mode == 0) Console.Write(map2[i, j] % 10);
                    if (mode == 1) Console.Write(map2[i, j] + " ");
                }
                Console.WriteLine();
            }
            Console.WriteLine();
#endif
        }

        public static void Draw1D(int[] array, int mode = 0)
        {
#if DEBUG
            Console.WriteLine(string.Join(" ", array));
#endif
        }
    }
}


Measured memory_bytes: 27744