using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using static System.Console;
using static System.Math;
using static MyIO;
using static MyUtil;

public class E
{
	public static void Main() => (new Solver()).Solve();
}

public class Solver
{
	public void Solve()
	{
		long N = GetLong();
		long K = GetLong();
		long[] A = GetLongs(N);
		Array.Sort(A);

		long[] low = new long[N];
		for(long i = 0; i < N; i++)
			low[i] = i;

		long[] high = new long[N];
		for(long i = 0; i < N; i++)
			high[i] = N - 1 - i;
	
		var mct = new ModCombTable((int)N, (long)1e9 + 7);
		var ans = new ModInt(0);
		for(long i = N - 1; i >= 0; i--)
		{
			if(low[i] < K - 1)
				break;

			ans += new ModInt(mct.ModComb(low[i], K - 1)) * new ModInt(A[i]);
		}

		for(long i = 0; i < N; i++)
		{
			if(high[i] < K - 1)
				break;

			ans -= new ModInt(mct.ModComb(high[i], K - 1)) * new ModInt(A[i]);
		}

		WriteLine(ans.Val);
	}
}

public class ModCombTable
{
	private long[] f = null;
	private long[] fInv = null;
	private long mod;

	public ModCombTable(int N, long m)
	{
		mod = m;

		// calc factorial
		f = new long[N+1];
		f[0] = 1;
		for(int i = 1; i <= N; i++)
			f[i] = (f[i-1] * i) % mod;

		// calc factorial inverse
		fInv = new long[N+1];
		fInv[N] = ModInv(f[N]);
		for(int i = N; i > 0; i--)
			fInv[i-1] = (fInv[i] * i) % mod;
	}

	public long ModComb(long n, long k)
	{
		if(n < 0 || n < k)
			return 0;
		
		return (((f[n] * fInv[k]) % mod) * fInv[n-k]) % mod;
	}

	public long ModInv(long x)
	{
		return ModPow(x, mod - 2);
	}

	public long ModPow(long x, long n)
	{
		long res = 1;
		while(n > 0)
		{
			if((n & 1) == 1)
				res = (res * x) % mod;
			x = (x * x) % mod;
			n >>= 1;
		}
		return res;
	}
}

public struct ModInt
{
	public long Mod { get; private set; }
	public long Val { get; private set; }

	public ModInt(long v)
	{
		// must initialize (default: Mod = 0)
		Mod = 1000000007;
		Val = (v + Mod) % Mod;
	}

	public static ModInt operator+(ModInt x, ModInt y)
	{
		long mod = x.Mod;
		long tmp = x.Val + y.Val;
		if(tmp >= mod)
			tmp -= mod;
		return new ModInt(tmp);
	}


	public static ModInt operator-(ModInt x, ModInt y)
	{
		long mod = x.Mod;
		long tmp = x.Val;
		if(tmp < y.Val)
			tmp += mod;
		tmp -= y.Val;
		return new ModInt(tmp);
	}

	public static ModInt operator*(ModInt x, ModInt y)
	{
		long mod = x.Mod;
		return new ModInt((x.Val * y.Val) % mod);
	}

	public static ModInt operator/(ModInt x, ModInt y)
	{
		long mod = x.Mod;
		long pow = mod - 2;
		long yy = y.Val;
		long inv = 1;
		while(pow > 0)
		{
			if((pow & 1) == 1)
				inv = (inv * yy) % mod;
			yy = (yy * yy) % mod;
			pow >>= 1;
		}
		return new ModInt((x.Val * inv) % mod);
	}

	public static ModInt operator+(ModInt x, long y) => x + new ModInt(y);
	public static ModInt operator-(ModInt x, long y) => x - new ModInt(y);
	public static ModInt operator*(ModInt x, long y) => x * new ModInt(y);
	public static ModInt operator/(ModInt x, long y) => x / new ModInt(y);
	public static ModInt operator++(ModInt x) => x + new ModInt(1);
	public static ModInt operator--(ModInt x) => x - new ModInt(1);
}

public static class MyIO
{
	private static string[] args = null;
	private static int num = -1;
	private static int used = -1;

	private static string getArg()
	{
		if(used == num)
		{
			args = Console.ReadLine().Split(' ');
			num = args.Length;
			used = 0;
		}
		return args[used++];
	}

	public static int GetInt() => int.Parse(getArg());
	public static long GetLong() => long.Parse(getArg());
	public static double GetDouble() => double.Parse(getArg());
	public static decimal GetDecimal() => decimal.Parse(getArg());
	public static string GetString() => getArg();
	public static char GetChar() => getArg()[0];
	public static int[] GetInts(long N) => Enumerable.Range(0, (int)N).Select(_ => GetInt()).ToArray();
	public static long[] GetLongs(long N) => Enumerable.Range(0, (int)N).Select(_ => GetLong()).ToArray();
	public static double[] GetDoubles(long N) => Enumerable.Range(0, (int)N).Select(_ => GetDouble()).ToArray();
	public static decimal[] GetDecimals(long N) => Enumerable.Range(0, (int)N).Select(_ => GetDecimal()).ToArray();
	public static string[] GetStrings(long N) => Enumerable.Range(0, (int)N).Select(_ => GetString()).ToArray();
}

public static class MyUtil
{
	public static void chmax<T>(ref T x, T y) where T : IComparable<T>
	{
		if(x.CompareTo(y) < 0)
			x = y;
	}

	public static void chmin<T>(ref T x, T y) where T : IComparable<T>
	{
		if(x.CompareTo(y) > 0)
			x = y;
	}

	public static void swap<T>(ref T x, ref T y)
	{
		T tmp = x;
		x = y;
		y = tmp;
	}
}

Measured memory_bytes: 31324