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

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

    static class E {
        private static int MAX = 400000;
        private static long[] xs = new long[MAX + 1];
        private static long[] ys = new long[MAX + 1];
        static long MOD = (int) 1e9 + 7;

        public void solve(int testNumber, MyScanner in, PrintWriter out) {
            int N = in.nextInt();
            int K = in.nextInt();
            int[] A = in.nextIntArray(N);

            Arrays.sort(A);

            long sum = 0;
            for (int i = K - 1; i < N; i++) {
                int ah = A[i];
                long ch = comb(i, K - 1) * ah;
                int al = A[N - i - 1];
                long cl = comb(i, K - 1) * al;
                sum += ch - cl;
                sum %= MOD;
            }
            out.println(sum);
        }

        private long comb(int n, int r) {
            if (r < 0 || r > n) return 0;
            if (xs[0] == 0) {
                xs[0] = 1;
                for (int i = 1; i <= MAX; i++) {
                    xs[i] = xs[i - 1] * i % MOD;
                }
                long inv = modpow(xs[MAX]);
                ys[MAX] = inv;
                for (int i = MAX; i >= 1; i--) {
                    inv = inv * i % MOD;
                    ys[i - 1] = inv;
                }
            }
            return xs[n] * (ys[r] * ys[n - r] % MOD) % MOD;
        }

        private long modpow(long a) {
            return modpow(a, MOD - 2);
        }

        private long modpow(long a, long n) {
            long r = 1;
            while (n > 0) {
                r = r * ((n % 2) != 0 ? a : 1) % MOD;
                a = a * a % MOD;
                n >>= 1;
            }
            return r;
        }

    }

    static class MyScanner {
        private BufferedReader in;
        private StringTokenizer st;

        public MyScanner(InputStream stream) {
            in = new BufferedReader(new InputStreamReader(stream));
        }

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

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

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

    }
}



Measured memory_bytes: 49072