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

#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define REP(i, n)  for(int i=0;i<(n);++i)
#define REPr(i, n) for(int i=(n)-1;i>=0; --i)
#define FORq(i, m, n) for(int i = (m);i <= (n);++i)
#define FORqr(i, m, n) for(int i = (n);i >=(m);--i)
#define PB push_back
#define MP make_pair
#define fst first
#define snd second
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x,0,sizeof(x))
#define FILL(x, y) memset(x,y,sizeof(x))
#define MM(x) memset(x,-1,sizeof(x))
#define ALL(x) (x).begin(),(x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y) cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG int x12345; cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
///////////////////////////////////////////////
const ll MOD = 1e9 + 7;
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
 
template<class T>
inline bool chmin(T &a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
 
template<class T>
inline bool chmax(T &a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}
 
template<int MOD>
struct Fp {
	long long val;
 
	constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
		if (val < 0) v += MOD;
	}
 
	constexpr int getmod() { return MOD; }
 
	constexpr Fp operator-() const noexcept {
		return val ? MOD - val : 0;
	}
 
	constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
 
	constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
 
	constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
 
	constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
 
	constexpr Fp &operator+=(const Fp &r) noexcept {
		val += r.val;
		if (val >= MOD) val -= MOD;
		return *this;
	}
 
	constexpr Fp &operator-=(const Fp &r) noexcept {
		val -= r.val;
		if (val < 0) val += MOD;
		return *this;
	}
 
	constexpr Fp &operator*=(const Fp &r) noexcept {
		val = val * r.val % MOD;
		return *this;
	}
 
	constexpr Fp &operator/=(const Fp &r) noexcept {
		long long a = r.val, b = MOD, u = 1, v = 0;
		while (b) {
			long long t = a / b;
			a -= t * b;
			swap(a, b);
			u -= t * v;
			swap(u, v);
		}
		val = val * u % MOD;
		if (val < 0) val += MOD;
		return *this;
	}
 
	constexpr bool operator==(const Fp &r) const noexcept {
		return this->val == r.val;
	}
 
	constexpr bool operator!=(const Fp &r) const noexcept {
		return this->val != r.val;
	}
 
	friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
		return os << x.val;
	}
 
	friend constexpr istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
		return is >> x.val;
	}
 
	friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
		if (n == 0) return 1;
		auto t = modpow(a, n / 2);
		t = t * t;
		if (n & 1) t = t * a;
		return t;
	}
};
 
using mint = Fp<MOD>;
 
void scdvec(vector<int> &a, int n, int s = 0) {
	for (int i = s; i < n; ++i) { scanf("%d", &a[i]); }
}
 
void sclldvec(vector<long long> &a, int n, int s = 0) {
	for (int i = s; i < n; ++i) { scanf("%lld", &a[i]); }
}
 
///////////////////////////////////////////////
///  ((((っ・ω・)っ |AC|

const int NMAX = 210000;
ll fac[NMAX],finv[NMAX],inv[NMAX];
void combinit(){
	fac[0] = fac[1] = 1;
	finv[0] = finv[1] = 1;
	inv[1] = 1;
	
	for(int i = 2; i <= NMAX-1; ++i){
		fac[i] = fac[i-1] * i % MOD;
		inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
		finv[i] = finv[i-1] * inv[i] % MOD;
	}
}

long long comb(int n,int r){
	if (n<r) return 0;
	if (n < 0 || r < 0) return 0;
	return fac[n] * (finv[r] * finv[n-r] % MOD) % MOD;
}

int main(){
	mint ans = 0;
	combinit();
	int N,K; cin >> N >> K;
	vector<ll> A(N+1);
	A[0] = -1e15;

	FORq(i,1,N){
		cin >> A[i];
	}

	sort(ALL(A));

	for(int x = 1; x <= N; x++){
		mint nans = A[x] * ((comb(x-1,K-1)) % MOD);
		ans += nans;
		nans = (A[x] * comb(N-x,K-1) ) % MOD;
		ans -= nans;
		//DB2(Mp-1,K-1);
	}

	cout << ans << endl;
}




Measured memory_bytes: 5888