#include<algorithm>
#include<bitset>
#include<cassert>
#include<cfloat>
#include<climits>
#include<cmath>
#include<deque>
#include<functional>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<string>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<vector>

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define llong long long
#define pb(a) push_back(a)

using namespace std;

typedef long long int ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;


template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) {
	return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}
template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T& t, const V& v) { t = v; }
template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T& t, const V& v) {
	for (auto& e : t) fill_v(e, v);
}


// aよりもbが大きいならばaをbで更新する
// (更新されたならばtrueを返す)
template <typename T>
bool chmax(T & a, const T & b) {
	if (a < b) {
		a = b;  // aをbで更新
		return true;
	}
	return false;
}
// aよりもbが小さいならばaをbで更新する
// (更新されたならばtrueを返す)
template <typename T>
bool chmin(T & a, const T & b) {
	if (a > b) {
		a = b;  // aをbで更新
		return true;
	}
	return false;
}


#define ARRAY_MAX 100005
const int INF = INT32_MAX / 3;
const ll MOD = 1e9 + 7;

int dx[4] = { 1,0,0,-1 };
int dy[4] = { 0,1,-1,0 };


/******************************************************************************************/


struct Combination {

	vector<ll> fact, rfact;

	Combination(ll sz) : fact(sz + 1), rfact(sz + 1) {

		fact[0] = 1;
		for (ll i = 1; i < fact.size(); i++) {
			fact[i] = fact[i - 1]%MOD * i % MOD;
		}
		//逆元
		rfact[sz] = inv(fact[sz]);
		for (ll i = sz - 1; i >= 0; i--) {
			rfact[i] = rfact[i + 1] %MOD * (i + 1) % MOD;
		}
	}

	ll inv(ll x) {
		return pow(x, MOD - 2);
	}
	ll pow(ll x, ll n) {
		ll ret = 1;
		while (n > 0) {
			if (n & 1) (ret *= x) %= MOD;
			(x *= x) %= MOD;
			n >>= 1;
		}
		return (ret);
	}
	ll P(ll n, ll r) {
		if (r < 0 || n < r) return (0);
		return (fact[n] * rfact[n - r] % MOD);
	}

	ll C(ll p, ll q) {
		if (q < 0 || p < q) return (0LL);
		return (fact[p]%MOD * rfact[q] % MOD * rfact[p - q] % MOD);
	}

	ll H(ll n, ll r) {
		if (n < 0 || r < 0) return (0);
		return (r == 0 ? 1 : C(n + r - 1, r));
	}
};



int main() {

	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed << setprecision(10);


	ll n, k;
	cin >> n >> k;


	Combination comb(n + 10);

	vector<ll> a(n);
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	sort(a.begin(), a.end());

	ll ans = 0;

	ll maxi = 0;

	ll mini = 0;
	//miniの足し算

	for (int i = 0; i < n; i++)
	{
		mini += (a[i] % MOD) * (comb.C(n - i - 1, k - 1) % MOD);
		mini = (mini + MOD) % MOD;
	}

	reverse(a.begin(), a.end());
	for (int i = 0; i < n; i++)
	{
		maxi += (a[i] % MOD) * (comb.C(n - i - 1, k - 1) % MOD);
		maxi = (maxi + MOD) % MOD;
		//maxi %= MOD;

	}


	


	

	cout << (maxi - mini + MOD) % MOD << endl;

	return 0;
}



Measured memory_bytes: 2560