import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map.Entry;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.StringTokenizer;

class Main {

    static final int MOD = 1000000007;
    static final int INF = 1 << 30;

    public static void main(String[] args) {
        FastScanner sc = new FastScanner(System.in);
        int N = sc.nextInt();
        int K = sc.nextInt();
        int[] a = sc.nextIntArray(N);
        Arrays.sort(a);
        combinationInit();
        long ansplus = 0;
        long ansminus = 0;
        for (int i = 0; i < N ; i++) {
            if (i >= K-1) {
                long com = combination(i, K - 1);
                long pls = a[i] * com;
                pls %= MOD;
                ansplus += pls;
                ansplus %= MOD;
            }
            if (i <= N - K) {
                long com = combination(N - i - 1 , K - 1);
                long pls = a[i] * com;
                pls %= MOD;
                ansminus += pls;
                ansminus %= MOD;
            }
            // System.out.println(ansplus+" "+ansminus);
        }
        System.out.println((MOD+ansplus-ansminus)%MOD);
    }

    static int MAX = 510000;
    static long[] fac = new long[MAX], finv = new long[MAX], inv = new long[MAX];

    static void combinationInit() {
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[1] = 1;
        for (int i = 2; i < MAX; i++) {
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[(int) MOD % i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }

    static long combination(int n, int k) {
        if (n < k)
            return 0;
        if (n < 0 || k < 0)
            return 0;
        return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
    }
}

class FastScanner {
    private BufferedReader reader = null;
    private StringTokenizer tokenizer = null;

    public FastScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
        tokenizer = null;
    }

    public String next() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public String nextLine() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

        return tokenizer.nextToken("\n");
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public double nextDouble() {
        return Double.parseDouble(next());
    }

    public ArrayList<Integer> nextIntList(int n) {
        ArrayList<Integer> a = new ArrayList<Integer>(n);
        for (int i = 0; i < n; i++)
            a.add(nextInt());
        return a;
    }

    public ArrayList<Long> nextLongList(int n) {
        ArrayList<Long> a = new ArrayList<Long>(n);
        for (int i = 0; i < n; i++)
            a.add(nextLong());
        return a;
    }

    public int[] nextIntArray(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = nextInt();
        return a;
    }

    public long[] nextLongArray(int n) {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = nextLong();
        return a;
    }
}

Measured memory_bytes: 54352