using System;
using System.Text;
using System.Collections.Generic;
using static System.Math;
using System.Linq;
using static System.Array;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var NK = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            var N = NK[0];
            var K = NK[1];

            var A = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
            Sort(A);

            var Max = new Modular(0);
            for (int i = (int)K-1; i < N; i++)
            {
                Max += A[i] * Modular.Ncr(i, K - 1);
            }

            Reverse(A);
            var Min = new Modular(0);
            for (int i = (int)K - 1; i < N; i++)
            {
                Min += A[i] * Modular.Ncr(i, K - 1);
            }
            var ans = Max - Min;
            Console.WriteLine(ans.value);
        }

        

    }

    class Modular
    {
        private const int M = 1000000007;
        public long value;
        public Modular(long value) { this.value = value; }
        public static implicit operator Modular(long a)
        {
            var m = a % M;
            return new Modular((m < 0) ? m + M : m);
        }
        public static Modular operator +(Modular a, Modular b)
        {
            return (a.value + b.value) % M;
        }
        public static Modular operator -(Modular a, Modular b)
        {
            return a.value - b.value;
        }
        public static Modular operator *(Modular a, Modular b)
        {
            return a.value * b.value;
        }
        public static Modular Pow(Modular a, int n)
        {
            switch (n)
            {
                case 0:
                    return 1;
                case 1:
                    return a;
                default:
                    var p = Pow(a, n / 2);
                    return p * p * Pow(a, n % 2);
            }
        }
        public static Modular operator /(Modular a, Modular b)
        {
            return a * Pow(b, M - 2);
        }
        private static readonly List<int> facs = new List<int> { 1 };
        private static Modular Fac(int n)
        {
            for (int i = facs.Count; i <= n; ++i)
            {
                facs.Add((int)(Math.BigMul(facs.Last(), i) % M));
            }
            return facs[n];
        }
        public static Modular Ncr(int n, int r)
        {
            return (n < r) ? 0
                 : (n == r) ? 1
                            : Fac(n) / (Fac(r) * Fac(n - r));
        }
        public static explicit operator int(Modular a)
        {
            return (int)a.value;
        }

        public static Modular Lcm(Modular a, Modular b)
        {
            var tmp = Gcdl(a.value, b.value);
            return ((a * b) / new Modular(tmp));
        }
        static Modular Gcd(Modular m, Modular n)
        {
            if (n.value > m.value) return Gcd(n, m);
            if (n.value == 0) return m;
            var r = new Modular(m.value % n.value);
            return Gcd(n, r);
        }

        static long Gcdl(long m, long n)
        {
            if (n > m) return Gcdl(n, m);
            if (n == 0) return m;
            return Gcdl(n, m % n);
        }
    }
}


Measured memory_bytes: 31452