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

#include <iostream>
#include <fstream>

#include <bits/stdc++.h>
#define se cout << "test ok" << endl;
#define sp << " " <<
#define enl "\n";
using namespace std;
using ll=long long;
using ld=long double;
ll mod = ll(1e9)+7;
ll inf = ll(1e18);
template<typename T1, typename T2> ll ceill(T1 a, T2 b){ return (a+b-1)/b; }
void ans(bool b, ostream& out=cout){ out << (b ? "Yes" : "No") << endl; }
void ans2(bool b, ostream& out=cout){ out << (b ? "YES" : "NO") << endl; }
template<typename T> inline bool chmax(T &a, T b){ if(a<b){a=b; return true;} return false; }
template<typename T> inline bool chmin(T &a, T b){ if(a>b){a=b; return true;} return false; }
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val){ fill((T*)array, (T*)(array+N), val); }
int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
int dy[] = {0, 1, 0, -1, 1, -1, -1, 1};

#ifndef INCLUDE_DEBUG_HPP
#define INCLUDE_DEBUG_HPP

#include <iostream>
#include <vector>
#include <map>
using namespace std;

namespace dbg {
    using ll = long long;

    class Debug {
    public:
        Debug(){}
        template<typename T>
        void vd(vector<T> vec);
        template<typename T>
        void vd2(vector<vector<T>> vec);
        template<typename T>
        void ad(T *arr, ll n);
        template<typename T, size_t N>
        void ad2(T (&arr)[N], ll n);
        template<typename T>
        void md(T mp);
    };

    template<typename T>
    void Debug::vd(vector<T> vec) {
        for (auto vv : vec) cout << vv << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::vd2(vector<vector<T>> vec) {
        for (auto vv : vec) for (auto vvv : vv) cout << vvv << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::ad(T *arr, ll n) {
        for (int i = 0; i <= n; i++) cout << arr[i] << " ";
        cout << endl;
    }

    template<typename T, size_t N>
    void Debug::ad2(T (&arr)[N], ll n) {
        for (int i = 0; i <= N; i++) for (int j = 0; j <= n; j++) cout << arr[i][j] << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::md(T mp) {
        cout << "key : value" << endl;
        for (auto const &m : mp) cout << m.first << " : " << m.second << endl;
        cout << endl;
    }

}

#endif //INCLUDE_DEBUG_HPP

using dbg::Debug;
Debug debug;

#ifndef DEFINE_COMBINATION_HPP
#define DEFINE_COMBINATION_HPP


using namespace std;

namespace comb {

    using ll = long long;
    ll mod = ll(1e9) + 7;

    class Comb {
    private:
        ll powm(ll a, ll b) {
            ll res = 1;
            while(b > 0){
                if(b & 1) (res *= a) %= mod;
                (a *= a) %= mod;
                b >>= 1;
            }
            return res;
        }

    public:
        vector<ll> fact;
        vector<ll> finv;
        vector<ll> inv;

        Comb() {};
        Comb(int n) { build(n); }
        void build(int n);
        ll res(int n, int k); // return nCk
        ll resone(ll n, ll k);
    };

    void Comb::build(int n) {
        inv.assign(n + 1, 0);
        fact.assign(n + 1, 0);
        finv.assign(n + 1, 0);
        fact[0] = fact[1] = finv[0] = finv[1] = 1;
        inv[1] = 1;
        for (int i = 2; i <= n; i++) {
            fact[i] = fact[i - 1] * i % mod;
            inv[i] = mod - inv[mod % i] * (mod / i) % mod;
            finv[i] = finv[i - 1] * inv[i] % mod;
        }
    }

    ll Comb::res(int n, int k) {
        if (n < k) return 0;
        if (n < 0 || k < 0) return 0;
        return fact[n] * (finv[k] * finv[n - k] % mod) % mod;
    }

    ll Comb::resone(ll n, ll k) {
        ll res = 1;
        for (ll i = 0; i < k; i++) {
            (res *= (n-i)) %= mod;
            (res *= this->powm(i+1, mod-2)) %= mod;
        }
        return res;
    }

}

#endif //DEFINE_COMBINATION_HPP


// verify
// https://atcoder.jp/contests/abc034/tasks/abc034_c

using comb::Comb;

class EMaxMinSums {
public:
	void solve(std::istream& in, std::ostream& out) {
	    int n, k;
	    in >> n >> k;
	    vector<ll> a(n);
	    int one = 0;
	    for(int i=0; i<n; i++){
	        in >> a[i];
	    }

	    vector<ll> b = a;

	    Comb comb(n);

	    ll res = 0;
        sort(a.begin(), a.end());
        sort(b.begin(), b.end(), greater<ll>());
	    for(int i=0; i<n; i++){
            res += comb.res(n-(i+1), k-1) * b[i];
            res -= comb.res(n-(i+1), k-1) * a[i];
            res = (res+mod)%mod;
	    }

	    out << res << endl;
	}
};


int main() {
	EMaxMinSums solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 4224