using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Runtime.InteropServices;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using static System.Math;
using static Math2;
using static io;
using static Utils;

public class CompetitiveProgramming {
  public void Solve () {
    var o = Ints;
    var N = o[0];
    var K = o[1];
    var A = Longs;
    var fc = new Factorical (2 * N);
    Array.Sort (A);
    long ans = 0;
    for (int i = 0; i < N; ++i) {
      ans += A[i] * fc.Comb (i, K - 1);
      ans %= MOD;
    }
    A = A.Reverse ().ToArray ();
    for (int i = 0; i < N; ++i) {
      ans -= A[i] * fc.Comb (i, K - 1);
      ans %= MOD;
    }
    Put (ans);

  }
}

public class Factorical {
  public long[] Fact;
  public long[] invFact;
  public Factorical (long N, long MOD = 1000000007) {
    Fact = new long[N + 1];
    invFact = new long[N + 1];
    Fact[0] = 1;
    Fact[1] = 1;
    for (long i = 2; i <= N; ++i) {
      Fact[i] = Fact[i - 1] * i % MOD;
      Fact[i] %= MOD;
    }
    invFact[N] = Power (Fact[N], MOD - 2);
    for (long i = N; i > 0; --i) {
      invFact[i - 1] = invFact[i] * i % MOD;
      invFact[i - 1] %= MOD;
    }
  }
  public long Comb (long n, long r, long MOD = 1000000007) {
    if (n < 0 || r < 0 || n < r) return 0;
    return Fact[n] * invFact[r] % MOD * invFact[n - r] % MOD;
  }
}

public static class Math2 {
  public const int INF = 1 << 29;
  public const long INFL = 1L << 60;
  public const long MOD = 1000000007;
  public const int MOD2 = 998244353;

  public static long Power (long a, long b, long MOD = 1000000007) //i^N
  {
    long res = 1;
    while (b > 0) {
      if ((b & 1) != 0) res = res * a % MOD;
      a = a * a % MOD;
      b >>= 1;
    }
    return res;
  }
  public static long Power2 (long a, long b) //i^N
  {
    long res = 1;
    while (b > 0) {
      if ((b & 1) != 0) res = res * a;
      a = a * a;
      b >>= 1;
    }
    return res;
  }
  public static long GCD (long a, long b) {
    while (b > 0) {
      var r = a % b;
      a = b;
      b = r;
    }
    return a;
  }
  public static long LCM (long a, long b) => (b / GCD (a, b)) * a;
  public static long Comb (long n, long r, int MOD = 1000000007) {
    if (r > n - r) r = n - r;
    long Nume = 1;
    long Deno = 1;

    if (r > n - r) r = n - r;
    for (long i = 1; i <= r; ++i) {
      Deno = (Deno * i) % MOD;
      Nume = Nume * (n - i + 1) % MOD;
    }
    return (Nume * inv (Deno)) % MOD;
  }
  public static long Comb2 (long n, long r) {
    long Nume = 1;
    long Deno = 1;

    if (r > n - r) r = n - r;
    for (long i = 1; i <= r; ++i) {
      Deno *= i;
      Nume *= n - i + 1;
    }
    return Nume / Deno;
  }
  public static long inv (long x, int MOD = 1000000007) {
    return Power (x, MOD - 2, MOD);
  }
}
public static class Utils {
  public static void Swap<T> (ref T A, ref T B) {
    T x = A;
    A = B;
    B = x;
  }
  public static int DigitSum (int N) {
    string s = N.ToString ();
    int ret = 0;
    for (int i = 0; i < s.Length; ++i) ret += s[i] - '0';
    return ret;
  }
  public static string ConvertBase (long N, int K) {
    StringBuilder ret = new StringBuilder ();
    while (N > 0) {
      var r = N % K;
      N /= K;
      ret.Append (r);
    }
    return new string (ret.ToString ().ToCharArray ().Reverse ().ToArray ());
  }
}
class Program {
  static void Main (string[] args) {
    var CompetitiveProgramming = new CompetitiveProgramming ();
    CompetitiveProgramming.Solve ();
  }
}
public static class io {
  public static string Str => Console.ReadLine ();
  public static string[] Strs => Str.Split (' ');
  public static long[] Longs => Strs.Select (long.Parse).ToArray ();
  public static int[] Ints => Strs.Select (int.Parse).ToArray ();
  public static char[] Chars => Str.ToArray ();
  public static double[] Doubles => Strs.Select (double.Parse).ToArray ();
  public static long Long1 => Longs[0];
  public static int Int1 => Ints[0];
  public static char Char1 => Chars[0];
  public static double Double1 => Doubles[0];
  public static long[] VerticalRead (int N) { long[] A = new long[N]; for (int i = 0; i < N; ++i) A[i] = Long1; return A; }
  public static void Put (string a) => Console.WriteLine (a);
  public static void Put (params object[] i) => Put (string.Join (" ", i));
  public static void Put<T> (IEnumerable<T> a) => Put (string.Join (" ", a));
  public static void VerticalPut<T> (IEnumerable<T> a) { foreach (var z in a) Put (z); }
  public static void YN (bool i) {
    if (i) Put ("Yes");
    else Put ("No");
  }
}

Measured memory_bytes: 32988