#include "bits/stdc++.h"
using namespace std;

long long MOD = 1000000007;

vector<long long> F, RF;

void init(long long N) {
	F.resize(N + 1), RF.resize(N + 1);
	vector<long long> inv(N + 1);
	inv[1] = 1;
	F[0] = F[1] = RF[0] = RF[1] = 1;
	for (int i = 2; i <= N; i++) {
		F[i] = (F[i - 1] * i) % MOD;
		inv[i] = MOD - (inv[MOD % i] * (MOD / i)) % MOD;
		RF[i] = (RF[i - 1] * inv[i]) % MOD;
	}
}

long long Comb(long long A, long long B) {
	if (B < 0 || A < B) return 0;
	return (F[A] * ((RF[A - B] * RF[B]) % MOD)) % MOD;
}

int main() {
	long long N, K, MAXS = 0, MINS = 0;
	cin >> N >> K;
	init(N);
	vector<long long> A(N);
	for (int i = 0; i < N; i++) cin >> A[i];
	sort(A.begin(), A.end());
	for (int i = 0; i < N; i++) {
		MAXS = (MAXS + Comb(i, K - 1) * A[i]) % MOD;
	}
	reverse(A.begin(), A.end());
	for (int i = 0; i < N; i++) {
		MINS = (MINS + Comb(i, K - 1) * A[i]) % MOD;
	}
	cout << (MOD + MAXS - MINS) % MOD << endl;
}