/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author
 */

#include <iostream>
#include <fstream>

#include <iostream>
#include "bits/stdc++.h"

using namespace std;

typedef unsigned long long ll;
typedef int integer;
//#define int                     long long
#define ii                      pair<int,int>
#define vii                     vector<pair<int,int>>
#define vi                      vector<int>
#define vvi                     vector<vector<int>>
#define f(a, b, c)                for(int a=b;a<c;a++)
#define s(x)                    in>>x
#define s2(x, y)                 in>>x>>y
#define s3(x, y, z)               in>>x>>y>>z
#define s4(x, y, z, w)             in>>x>>y>>z>>w
#define p(x)                    out<<x<<endl
#define p2(x, y)                 out<<x<<" "<<y<<endl
#define pl(x)                   out<<x
#define p2l(x, y)                out<<x<<" "<<y
#define p1d(a, n)                for(int ix=0;ix<n;ix++) out<<a[ix]<<" "; out<<endl
#define p2d(a, n, m)              for(int ix=0;ix<n;ix++){ for(int jx=0;jx<m;jx++) out<<a[ix][jx]<<" "; out<<endl;}
#define range(v)                v.begin(),v.end()
#define w(x)                    while(x--)
#define rst(x)                  memset(x, -1, sizeof(x))
#define UNVISITED               -1
#define VISITED                 0
#define EXPLORED                2
const int MOD = 1e9 + 7;

struct mod_int {
    int val;

    mod_int(long long v = 0) {
        if (v < 0) v = v % MOD + MOD;
        if (v >= MOD) v %= MOD;
        val = v;
    }

    static int mod_inv(int a, int m = MOD) {
        // https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Example
        int g = m, r = a, x = 0, y = 1;

        while (r != 0) {
            int q = g / r;
            g %= r;
            swap(g, r);
            x -= q * y;
            swap(x, y);
        }

        return x < 0 ? x + m : x;
    }

    explicit operator int() const {
        return val;
    }

    mod_int &operator+=(const mod_int &other) {
        val += other.val;
        if (val >= MOD) val -= MOD;
        return *this;
    }

    mod_int &operator-=(const mod_int &other) {
        val -= other.val;
        if (val < 0) val += MOD;
        return *this;
    }

    static unsigned fast_mod(uint64_t x, unsigned m = MOD) {
#if !defined(_WIN32) || defined(_WIN64)
        return x % m;
#endif
        // Optimized mod for Codeforces 32-bit machines.
        // x must be less than 2^32 * m for this to work, so that x / m fits in a 32-bit integer.
        unsigned x_high = x >> 32, x_low = (unsigned) x;
        unsigned quot, rem;
        asm("divl %4\n"
        : "=a" (quot), "=d" (rem)
        : "d" (x_high), "a" (x_low), "r" (m));
        return rem;
    }

    mod_int &operator*=(const mod_int &other) {
        val = fast_mod((uint64_t) val * other.val);
        return *this;
    }

    mod_int &operator/=(const mod_int &other) {
        return *this *= other.inv();
    }

    friend mod_int operator+(const mod_int &a, const mod_int &b) { return mod_int(a) += b; }

    friend mod_int operator-(const mod_int &a, const mod_int &b) { return mod_int(a) -= b; }

    friend mod_int operator*(const mod_int &a, const mod_int &b) { return mod_int(a) *= b; }

    friend mod_int operator/(const mod_int &a, const mod_int &b) { return mod_int(a) /= b; }

    mod_int &operator++() {
        val = val == MOD - 1 ? 0 : val + 1;
        return *this;
    }

    mod_int &operator--() {
        val = val == 0 ? MOD - 1 : val - 1;
        return *this;
    }

    mod_int operator++(int) {
        mod_int before = *this;
        ++*this;
        return before;
    }

    mod_int operator--(int) {
        mod_int before = *this;
        --*this;
        return before;
    }

    mod_int operator-() const {
        return val == 0 ? 0 : MOD - val;
    }

    bool operator==(const mod_int &other) const { return val == other.val; }

    bool operator!=(const mod_int &other) const { return val != other.val; }

    mod_int inv() const {
        return mod_inv(val);
    }

    mod_int pow(long long p) const {
        assert(p >= 0);
        mod_int a = *this, result = 1;

        while (p > 0) {
            if (p & 1)
                result *= a;

            a *= a;
            p >>= 1;
        }

        return result;
    }

    friend ostream &operator<<(ostream &stream, const mod_int &m) {
        return stream << m.val;
    }
};

mod_int facts[100007];

mod_int comb(int n, int k) {
    return facts[n] / (facts[k] * facts[n - k]);
}

class EMaxMinSums {
public:
    void solve(std::istream &in, std::ostream &out) {

        facts[0] = 1;
        f(i, 1, 100007) {
            facts[i] = i * facts[i - 1];
        }
        int n, k;
        s2(n, k);
        map<int, int> elements;
        f(i, 0, n) {
            int buf;
            s(buf);
            if (elements.count(buf)) elements[buf]++;
            else elements[buf] = 1;
        }
        mod_int maxs = 0;
        int x = n;
        auto p = elements.rbegin();
        while (x >= k && p != elements.rend()) {
            int v = p->first;
            int hm = p->second;
            f(i,0,hm){
                if(x-i>=k) maxs+=v*comb(x-1-i,k-1);
            }
            x -= hm;
            p++;
        }
        mod_int mins = 0;
        x = n;
        auto pp = elements.begin();
        while (x >= k && pp != elements.end()) {
            int v = pp->first;
            int hm = pp->second;
            f(i,0,hm){
                if(x-i>=k) mins+=v*comb(x-1-i,k-1);
            }
            x -= hm;
            pp++;
        }
        out<<maxs - mins<<endl;
    }
};


integer main() {
	EMaxMinSums solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
}


Measured memory_bytes: 4992