#include<iostream>
#include<iomanip>
#include<map>
#include<unordered_map>
#include<set>
#include<vector>
#include<algorithm>
#include<cmath>
#include<string>
#include<queue>
#include<stack>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> pii;
typedef pair<ll, bool> pib;

typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;

typedef vector<double> vf;

typedef long double ld;

#define mp make_pair
#define pb push_back

#define rep(i, N) for(ll i = 0; i < N; i++)
#define repa(i, a, N) for(ll i = a; i < N; i++)
#define repr(i, N) for(ll i = N-1; i >= 0; i--)

#define drep(i, N, j, M) rep(i, N) rep(j, M)
#define trep(i, N, j, M, k, L) rep(i, N) rep(j, M) rep(k, L)

#define all(A) A.begin(), A.end()

#define printItr(A) for(auto itr = A.begin(); itr != A.end(); ++itr){cout << * itr << ", ";}cout << endl;

const double PI = 3.141592653589793;

const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;

const ll LLMAX = numeric_limits<long long>::max();

void No() {
	cout << "No" << endl;
}

void Yes() {
	cout << "Yes" << endl;
}

ll id(char c) {
	return c - 'a';
}

bool isOK(vi& a, ll k, ll index) {
	return a[index] < k * (index + 1);
}

ll binary_search(vi& a, ll k) {
	ll left = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
	ll right = (ll)a.size(); // 「index = a.size()-1」が条件を満たさないこともあるので、初期値は a.size()

	/* どんな二分探索でもここの書き方を変えずにできる！ */
	while (right - left > 1) {
		ll mid = left + (right - left) / 2;

		if (isOK(a, k, mid))
			right = mid;
		else left = mid;
	}

	/* left は条件を満たさない最大の値、right は条件を満たす最小の値になっている */
	return right;
}

template<class t, class u>
ostream& operator<<(ostream& os, const pair<t, u>& p) {
	return os << "{" << p.first << "," << p.second << "}";
}

template<class t> ostream& operator<<(ostream& os, const vector<t>& v) {
	os << "{";
	for (auto e : v)os << e << ",";
	return os << "}";
}

bool eq(vi P, vi Q) {
	rep(i, P.size()) {
		if (P[i] != Q[i]) {
			return false;
		}
	}
	return true;
}

ll gcd(ll a, ll b)
{
	if (a%b == 0)
	{
		return(b);
	}
	else
	{
		return(gcd(b, a%b));
	}
}
ll lcm(ll a, ll b)
{
	return a * b / gcd(a, b);
}

ll modinv(ll a, ll m) {
	ll b = m, u = 1, v = 0;
	while (b) {
		ll t = a / b;
		a -= t * b; swap(a, b);
		u -= t * v; swap(u, v);
	}
	u %= m;
	if (u < 0) u += m;
	return u;
}

ll H, W;
bool inrange(ll h, ll w) {
	if (h < 0 || h >= H || w < 0 || w >= W) {
		return false;
	}
	return true;
}

ll toS(ll h, ll w) {
	return h * W + w;
}

// グラフの最短距離列挙
void warshall_floyd(ll n, vvi &d) {
	trep(k, n, i, n, j, n) {
		d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
	}
}

vector<ll> fac(1e6+1); //n!(mod M)
vector<ll> ifac(1e6+1); //k!^{M-2} (mod 
ll comb(ll a, ll b) { //aCbをmod計算
	if (a == 0 && b == 0)return 1;
	if (a < b || a < 0)return 0;
	ll tmp = ifac[a - b] * ifac[b] % MOD;
	return tmp * fac[a] % MOD;
}
ll mpow(ll x, ll n) { //x^n(mod M) ←普通にpow(x,n)では溢れてしまうため，随時mod計算
	ll ans = 1;
	while (n != 0) {
		if (n & 1) ans = ans * x % MOD;
		x = x * x % MOD;
		n = n >> 1;
	}
	return ans;
}

int main() {
	ll N, K;
	cin >> N >> K;

	vi A(N);

	rep(i, N) {
		cin >> A[i];
	}

	sort(all(A));

	//大した量ではないので，先にfax[i]とifax[i]を全て計算しておく
	fac[0] = 1;
	ifac[0] = 1;
	for (ll i = 0; i < 1e6; i++) {
		fac[i + 1] = fac[i] * (i + 1) % MOD; // n!(mod M)
		ifac[i + 1] = ifac[i] * mpow(i + 1, MOD - 2) % MOD; // k!^{M-2} (mod M) ←累乗にmpowを採用
	}

	ll m = 0;
	ll M = 0;
	rep(i, N - K + 1) {
		m += (comb(N - i - 1, K - 1) * A[i]) % MOD;
		m %= MOD;
	}

	rep(i, N - K + 1) {
		M += (comb(N - i - 1, K - 1) * A[N-1 - i]) % MOD;
		M %= MOD;
	}

	cout << (((M - m) % MOD) + MOD) % MOD << endl;

	return 0;
}


Measured memory_bytes: 16640