using System;
using System.Collections.Generic;
using System.Linq;
using static Constans;

public static class Constans
{
    public const long Mod = (long)1e9 + 7;
}

public class Combination
{
    private readonly long[] Fact;
    private readonly long[] InvFact;  // モジュラ逆数

    public Combination(long n)
    {
        Fact = new long[n + 1];
        InvFact = new long[n + 1];

        long f = 1;
        Fact[0] = f;
        for (long i = 1; i <= n; i++)
        {
            f *= i;
            f %= Mod;
            Fact[i] = f;
        }

        long inv = ModInt.GetInv(f).Num;

        InvFact[n] = inv;
        for (long i = n - 1; i >= 0; i--)
        {
            InvFact[i] = (InvFact[i + 1] * (i + 1)) % Mod;
        }
    }

    // nCr
    public ModInt GetCombination(long n, long r)
    {
        var comb = Fact[n] * InvFact[r] % Mod;
        comb = comb * InvFact[n - r] % Mod;
        return new ModInt(comb);
    }
}

public struct ModInt
{
    public long Num;

    public ModInt(long num)
    {
        Num = num;
    }

    // x^n
    public static long Power(long x, long n)
    {
        long pow = 1;
        while (n > 0)
        {
            if (n % 2 == 0)
            {
                x *= x;
                x %= Mod;
                n /= 2;
            }
            else
            {
                pow *= x;
                pow %= Mod;
                n--;
            }
        }

        return pow;
    }

    public static ModInt GetInv(long f)
    {
        // フェルマーの小定理 a^(-1) = a^(mod-2)
        return new ModInt(Power(f, Mod - 2));
    }

    public ModInt GetInv()
    {
        return GetInv(Num);
    }

    public override string ToString()
    {
        return Num.ToString();
    }

    public static ModInt operator +(ModInt l, ModInt r)
    {
        l.Num += r.Num;
        if (l.Num >= Mod)
        {
            l.Num -= Mod;
        }
        return l;
    }

    public static ModInt operator -(ModInt l, ModInt r)
    {
        l.Num -= r.Num;
        if (l.Num < 0)
        {
            l.Num += Mod;
        }
        return l;
    }

    public static ModInt operator *(ModInt l, ModInt r)
    {
        return new ModInt(l.Num * r.Num % Mod);
    }

    public static implicit operator ModInt(long n)
    {
        n %= Mod;
        if (n < 0)
        {
            n += Mod;
        }
        return new ModInt(n);
    }
}

public class Solution
{
    public static void Main()
    {
        var vals = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        var N = vals[0];
        var K = vals[1];
        var A = Console.ReadLine().Split(' ').Select(long.Parse).OrderBy(v => v).ToArray();
        var acc = new ModInt[N + 1];
        for (int i = 1; i <= N; i++)
        {
            acc[i] = acc[i - 1] + A[i - 1];
        }

        if (K == 1)
        {
            Console.WriteLine(0);
            return;
        }

        var comb = new Combination(N);
        var sum = new ModInt(0);
        /*
        for (int right = N - 1; right >= 0; right--)
        {
            var max = A[right];
            for (int left = 0; left + K - 1 <= right; left++)
            {
                var f = (ModInt)(max - A[left]);
                sum += f * comb.GetCombination(right - left + 1 - 2, K - 2);
            }
        }
        */
        for (int w = K; w <= N; w++)
        {
            var len = N - w;
            var f = acc[N] - acc[N - len - 1] - (acc[N - w + 1]);
            sum += f * comb.GetCombination(w - 2, K - 2);
        }

        Console.WriteLine(sum);
    }
}

Measured memory_bytes: 31668