import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author anand.oza
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        EMaxMinSums solver = new EMaxMinSums();
        solver.solve(1, in, out);
        out.close();
    }

    static class EMaxMinSums {
        NumberTheory.Mod107 mod = new NumberTheory.Mod107();

        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt(), k = in.nextInt();
            Integer[] a = in.readIntegerArray(n);
            Arrays.sort(a);

            for (int i = 0; i < n; i++) {
                a[i] = Math.toIntExact(mod.normalize(a[i]));
            }

            long answer = 0;
            for (int i = 0; i < n; i++) {
                long pr = mod.div(mod.ncr(i, k - 1), mod.ncr(n, k));
                answer = mod.add(answer, mod.mult(pr, a[i]));
            }
            Util.reverse(a);
            for (int i = 0; i < n; i++) {
                long pr = mod.div(mod.ncr(i, k - 1), mod.ncr(n, k));
                answer = mod.subtract(answer, mod.mult(pr, a[i]));
            }

            answer = mod.mult(answer, mod.ncr(n, k));

            out.println(answer);
        }

    }

    static class Util {
        public static <T> void swap(T[] x, int i, int j) {
            T t = x[i];
            x[i] = x[j];
            x[j] = t;
        }

        public static <T> void reverse(T[] x) {
            for (int i = 0, j = x.length - 1; i < j; i++, j--) {
                swap(x, i, j);
            }
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

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

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

        public Integer[] readIntegerArray(int n) {
            Integer[] x = new Integer[n];
            for (int i = 0; i < n; i++) {
                x[i] = nextInt();
            }
            return x;
        }

    }

    static class NumberTheory {
        private static void ASSERT(boolean assertion) {
            if (!assertion)
                throw new AssertionError();
        }

        public abstract static class Modulus<M extends NumberTheory.Modulus<M>> {
            ArrayList<Long> factorial = new ArrayList<>();
            ArrayList<Long> invFactorial = new ArrayList<>();

            public abstract long modulus();

            public Modulus() {
                super();
                factorial.add(1L);
                invFactorial.add(1L);
            }

            public long fact(int n) {
                while (factorial.size() <= n) {
                    factorial.add(mult(factorial.get(factorial.size() - 1), factorial.size()));
                }

                return factorial.get(n);
            }

            public long fInv(int n) {
                while (invFactorial.size() <= n) {
                    invFactorial.add(div(invFactorial.get(invFactorial.size() - 1), invFactorial.size()));
                }

                return invFactorial.get(n);
            }

            public long ncr(int n, int r) {
                ASSERT(n >= 0);
                if (r < 0 || n < r)
                    return 0;
                return mult(fact(n), mult(fInv(r), fInv(n - r)));
            }

            public long normalize(long x) {
                x %= modulus();
                if (x < 0)
                    x += modulus();
                return x;
            }

            public long add(long a, long b) {
                long v = a + b;
                return v < modulus() ? v : v - modulus();
            }

            public long subtract(long a, long b) {
                long v = a - b;
                return v < 0 ? v + modulus() : v;
            }

            public long mult(long a, long b) {
                return (a * b) % modulus();
            }

            public long div(long a, long b) {
                return mult(a, inv(b));
            }

            public long inv(long value) {
                long g = modulus(), x = 0, y = 1;
                for (long r = value; r != 0; ) {
                    long q = g / r;
                    g %= r;

                    long temp = g;
                    g = r;
                    r = temp;

                    x -= q * y;

                    temp = x;
                    x = y;
                    y = temp;
                }

                ASSERT(g == 1);
                ASSERT(y == modulus() || y == -modulus());

                return normalize(x);
            }

        }

        public static class Mod107 extends NumberTheory.Modulus<NumberTheory.Mod107> {
            public long modulus() {
                return 1_000_000_007L;
            }

        }

    }
}



Measured memory_bytes: 45580