//#include "pch.h"
#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <limits.h>
#include <string>
#include <deque>
#include <cmath>
#include <bitset>
#include <time.h>
#include <stdio.h>

using namespace std;
typedef long long ll;
typedef pair<int, ll> P;

#define rip(i, n, s) for (int i = (s);i < (int)( n ); i++)
#define mpa make_pair
#define all(a) a.begin(), a.end()

template<typename T>
void vdeb(vector<T> &da) {
	int size = da.size();
	for (int i = 0;i < size;i++) {
		if (i == size - 1) {
			cout << da[i] << endl;
		}
		else {
			cout << da[i] << ' ';
		}
	}
}
template<typename T>
void vvdeb(vector<T> &da) {
	int size = da.size();
	for (int i = 0;i < size;i++) {
		vdeb(da[i]);
	}
}
long long pow(long long n, long long p, long long k) {//n^k(mod p)
	if (!k) return 1;
	long long a = pow(n,p, k>>1);
	a = a * a%p;
	if (k & 1) a = a * n%p;
	return a;
}
void euclid(long long &a, long long &b, long long p) { // a>=b A*b+B*(a-a/b*b)=1
	if (a == 1) {
		a = 1;
	}
	else {
		long long A = b, B = a % b;
		euclid(A, B, p);
		b = (A - (p + a / b) % p * B % p + p) % p;
		a = B;
	}
}
long long rev(long long n, long long p) {//nの逆元 n*x-p*y=1
	//long long q = p;
	//euclid(p, n, p);
	//return n % q;
    return pow(n,p,p-2);
}
long long bino(long long n, long long m, long long p) {//nCm(mod p)
	long long ans = 1, div = 1;
	for(int i = 0;i < m; i++){
		ans = (n - i) * ans % p;
		div = div * (i +1) % p;
	}
	return ans * rev(div, p) % p;
}
struct modint {
	long long num;
	long long p;
	modint() {
		num = 0;
		p = 1000000007;
	}
	modint(int x) {
		p = 1000000007;
		num = (long long)x%p;
	}
	modint(long long x) {
		p = 1000000007;
		num = x % p;
	}
	modint operator+(const modint &other) {
		modint ret;
		ret.p = p;
		ret.num = (num + other.num) % p;
		return ret;
	}
	modint operator+(const long long &other) {
		modint ret;
		ret.p = p;
		ret.num = (num + (other%p)) % p;
		return ret;
	}
	modint operator-(const modint &other) {
		modint ret;
		ret.p = p;
		ret.num = (num - other.num + p) % p;
		return ret;
	}
	modint operator-(const long long &other) {
		modint ret;
		ret.p = p;
		ret.num = (num - (other%p) + p) % p;
		return ret;
	}
	modint operator*(const modint &other) {
		modint ret;
		ret.p = p;
		ret.num = (num*other.num) % p;
		return ret;
	}
	modint operator*(const long long &other) {
		modint ret;
		ret.p = p;
		ret.num = (num*(other%p)) % p;
		return ret;
	}
	modint operator/(const modint &other) {
		modint ret;
		ret.p = p;
		ret.num = (num*rev(other.num, p)) % p;
		return ret;
	}
	modint operator/(const long long &other) {
		modint ret;
		ret.p = p;
		ret.num = (num*rev(other%p, p)) % p;
		return ret;
	}
	void operator+=(const long long &other) {
		num = (num + other) % p;
	}
	void operator+=(const modint &other) {
		num = (num + other.num) % p;
	}
	void operator-=(const long long &other) {
		num = (num - other + p) % p;
	}
	void operator-=(const modint &other) {
		num = (num - other.num + p) % p;
	}
	void operator*=(const long long &other) {
		num = (num*other) % p;
	}
	void operator*=(const modint &other) {
		num = (num*other.num) % p;
	}
};

int main(){
	int n, k;cin >> n >> k;
	vector<ll> da(n);
	rip(i,n,0) cin >> da[i];
	sort(all(da));
	modint ans, c(1);
	if(k==1){
		cout << 0 << endl;
		return 0;
	}
	rip(i,n,k-1){
		ans += c*(da[i] - da[n-i-1]);
		c *= i+1;
		c = c/(i-k+2);
	}
	cout << ans.num << endl;
}

Measured memory_bytes: 1024