/*
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@TT_beginner
*/
#include "bits/stdc++.h"
using namespace std;
#define ok1 printf("ok1\n")
#define ok2 printf("ok2\n")
#define M LLONG_MAX
#define rep(i,n) for(int i=0;i<n;++i)
#define REP(i,s,n) for(int i=(s);i<(n);++i)
#define repr(i,n) for(int i=n-1;i>=0;--i)
#define REPR(i,s,n) for(int i=(s);i>=(n);--(i))
#define all(a) (a).begin(),(a).end()
#define reall(a) (a).rbegin(),(a).rend()
#define pb emplace_back //emplace_backの方が速いが使い慣れてないため
#define DOUBLE fixed << setprecision(15)
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
const double pi = acos(-1.0L);
typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef deque<ll> dll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> TLL;
typedef vector<P> vP;
typedef vector<TLL> vTLL;
const ll mod = 1e9 + 7;
//const ll mod = 998244353;
ll dy[4] = { 1,0,-1,0 };
ll dx[4] = { 0,1,0,-1 };
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class A, class B>
ostream& operator<<(ostream& ost, const pair<A, B>&p) {
	ost << "{" << p.first << ", " << p.second << "} ";
	return ost;
}
template<class T>
ostream& operator<<(ostream& ost, const vector<T>&v) {
	ost << "{";
	for (int i = 0; i<v.size(); i++) {
		if (i)ost << ", ";
		ost << v[i];
	}
	ost << "} ";
	return ost;
}
template<class A, class B>
ostream& operator<<(ostream& ost, const map<A, B>&v) {
	ost << "{";
	for (auto p:v) {
		ost << "{" << p.first << ", " << p.second << "} ";
	}
	ost << "} ";
	return ost;
}
bool out_check(ll a, ll b) { return (0 <= a && a < b); }
double pitagoras(ll a, ll b, ll c, ll d) {
	double dx = a - b, dy = c - d;
	return pow(dx * dx + dy * dy, 0.5);
}
ll modpow(ll a, ll b) { ll c = 1; while (b > 0) { if (b & 1) { c = a * c%mod; }a = a * a%mod; b >>= 1; }return c; }
void Yes(bool x) { cout << ((x) ? "Yes\n" : "No\n"); }
void YES(bool x) { cout << ((x) ? "YES\n" : "NO\n"); }
void yes(bool x) { cout << ((x) ? "yes\n" : "no\n"); }
void Yay(bool x) { cout << ((x) ? "Yay!\n" : ":(\n"); }
vector<string> DAY = { "SUN", "MON", "TUE", "WED", "THU", "FRI", "SAT" };
ll  n, m, d, e, r, l, k, h, Q, ans, ret = M;
bool flag = false, flag2 = false, flag3 = false;
string s, t, u;

ll factorial[20000500];
void make_factorial_table() {
	factorial[0] = 1;
	for (int i = 1; i < 20000500; i++) {
		factorial[i] = factorial[i - 1] * i%mod;
	}
}

ll bisection_power(ll x, ll y) {
	if (y == 0) { return 1; }
	ll t = bisection_power(x*x%mod, y / 2);
	if (y % 2 == 1) { t = t * x%mod; }
	return t;
}

ll nCr(int n, int r) {
	if (n<r || n <= 0 || r<0) { return 0; }
	ll a = factorial[n];
	ll b = a * bisection_power(factorial[r], (mod - 2)) % mod;
	ll c = b * bisection_power(factorial[n - r], mod - 2) % mod;
	return c % mod;
}

int main() {
	cin.tie(0); ios::sync_with_stdio(false);//not intractive
	make_factorial_table();
	cin >> n >> k;
	vll a(n); rep(i, n)cin >> a[i];
	sort(all(a));
	rep(i, n-k+1) {
		//cout << n - i - 1 <<" " << nCr(n - i - 1, k-1) << endl;
		//cout << -(a[i])*(nCr(n - i - 1, k-1)) << endl;
		ans += -(a[i])*(nCr(n - i - 1, k-1));
		ans %= mod;
	}
	//cout << ans << endl;
	REP(i, k-1, n) {
		//cout << i<< " "<< (nCr(i, k-1)) << endl;
		//cout << a[i] * (nCr(i,k-1)) << endl;
		ans += a[i] * (nCr(i, k-1));
		ans %= mod;
	}
	cout << ans << endl;
}


Measured memory_bytes: 157312