//include
//------------------------------------------
#include <bits/stdc++.h>
using namespace std;

//typedef
//------------------------------------------
typedef int64_t LL;
typedef vector<LL> VL;
typedef vector<VL> VVL;
typedef vector<string> VS;
typedef pair<LL, LL> PLL;

//container util
//------------------------------------------
#define whole(f,x,...) ([&](decltype((x)) whole) { return (f)(begin(whole), end(whole), ## __VA_ARGS__); })(x)
#define rwhole(f,x,...) ([&](decltype((x)) whole) { return (f)(rbegin(whole), rend(whole), ## __VA_ARGS__); })(x)
#define EACH(i,c) for(decltype((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) whole(sort, c)
#define RSORT(c) rwhole(sort, c)

//constant
//--------------------------------------------
constexpr double EPS = 1e-10;
constexpr double PI  = M_PI;
constexpr int MOD = 1000000007;

// grid
//--------------------------------------------
VL dx = {0, 1, 0, -1};
VL dy = {1, 0, -1, 0};
VL dx2 = {-1, 0, 1, -1, 1, -1, 0, 1};
VL dy2 = {-1, -1, -1, 0, 0, 1, 1, 1};

//debug
//--------------------------------------------
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

//IO accelerate
//--------------------------------------------
struct InitIO {
	InitIO() {
		cin.tie(nullptr);
		ios_base::sync_with_stdio(false);
		cout << fixed << setprecision(30);
	}
} init_io;

//template
//--------------------------------------------
template<typename T>
istream& operator >>(istream& is, vector<T>& vec) {
	for(T& x: vec) is >> x;
	return is;
}
template<typename T>
ostream& operator <<(ostream& os, const vector<T>& vec) {
	for(int i=0; i<vec.size(); i++){
		os << vec[i] << ( i+1 == vec.size() ? "" : "\t" );
	}
	return os;
}
template<typename T>
ostream& operator <<(ostream& s, const vector<vector<T>>& vv) {
	for (int i = 0; i < vv.size(); ++i) {
		s << vv[i] << endl;
	}
	return s;
}

// 多重vector
// auto dp=make_v<int>(4,h,w) みたいに使える
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...));
}

// 多重vectorのためのfill
// fill_v(dp,0) みたいに使える
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);
}

template<class T,class U> void chmax(T&a,U b){if(a<b)a=b;}
template<class T,class U> void chmin(T&a,U b){if(b<a)a=b;}
LL gcd(LL a, LL b) { return b?gcd(b,a%b):a;}
LL lcm(LL a, LL b) { return a/gcd(a,b)*b;}

//main code
// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
// const int MOD = 1000000007;
struct mint {
	LL x; // typedef long long LL;
	constexpr mint(LL x=0):x((x%MOD+MOD)%MOD){}
	constexpr mint operator-() const { return mint(-x);}
	constexpr mint& operator+=(const mint& a) {
		if ((this->x += a.x) >= MOD) this->x -= MOD;
		return *this;
	}
	constexpr mint& operator-=(const mint& a) {
		if ((this->x += MOD-a.x) >= MOD) this->x -= MOD;
		return *this;
	}
	constexpr mint& operator*=(const mint& a) {
		(this->x *= a.x) %= MOD;
		return *this;
	}
	constexpr mint operator+(const mint& a) const {
		mint res(*this);
		return res+=a;
	}
	constexpr mint operator-(const mint& a) const {
		mint res(*this);
		return res-=a;
	}
	constexpr mint operator*(const mint& a) const {
		mint res(*this);
		return res*=a;
	}
	constexpr mint pow(LL t) const {
		if (!t) return 1;
		mint a = pow(t>>1);
		a *= a;
		if (t&1) a *= *this;
		return a;
	}

	// for prime MOD
	constexpr mint inv() const {
		return pow(MOD-2);
	}
	constexpr mint& operator/=(const mint& a) {
		return (*this) *= a.inv();
	}
	constexpr mint operator/(const mint& a) const {
		mint res(*this);
		return res/=a;
	}

	constexpr bool operator==(const mint& a) const {
		return this->x == a.x;
	}
	constexpr bool operator!=(const mint& a) const {
		return this->x != a.x;
	}
	constexpr bool operator<(const mint& a) const {
		return this->x < a.x;
	}

	friend istream& operator>>(istream& is, mint& m) {
		return is >> m.x;
	}
	friend ostream& operator<<(ostream& os, const mint& m) {
		return os << m.x;
	}
};
typedef vector<mint> VM;
typedef vector<VM> VVM;

// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
struct combination {
	vector<mint> fact, ifact;
	combination(int n):fact(n+1),ifact(n+1) {
		assert(n < MOD);
		fact[0] = 1;
		for (int i = 1; i <= n; ++i) fact[i] = fact[i-1]*i;
		ifact[n] = fact[n].inv();
		for (int i = n; i >= 1; --i) ifact[i-1] = ifact[i]*i;
	}
	mint operator()(int n, int k) {
		if (k < 0 || k > n) return 0;
		return fact[n]*ifact[k]*ifact[n-k];
	}
};

int main(int argc, char *argv[])
{
	int n,k;
	cin >> n >> k;
	VL a(n);
	cin >> a;
	SORT(a);
	combination cmb(n);
	mint ans = 0;
	for (int i = 0; i < 2; i++) {
		for (int j = 0; j < n; j++) {
			mint tmp = cmb(j, k-1);
			if (i == 1) {tmp = -tmp;}
			ans += tmp * a[j];
		}
		whole(reverse, a);
	}
	cout << ans << endl;
	return 0;
}


Measured memory_bytes: 2560