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

template<typename T, typename U>
using pv=vector<pair<T,U>>;
template<typename T>
using matrix=vector<vector<T>>;
template<typename T>
using pque=priority_queue<T>;
template<typename T>
using lpque=priority_queue<T,vector<T>,greater<T>>;

using ll=long long;
using intpair=pair<int,int>;
using llpair=pair<ll,ll>;
using ilpair=pair<int,ll>;
using lipair=pair<ll,int>;
using intvec=vector<int>;
using llvec=vector<ll>;
using intq=queue<int>;
using llq=queue<ll>;
using intmat=vector<intvec>;
using llmat=vector<llvec>;

#define PI 3.141592653589793
#define INTINF ((1<<30)-1)
#define LLINF ((1LL<<62)-1)
#define MPRIME 1000000007
#define MPRIME9 998244353
#define MMPRIME ((1ll<<61)-1)

#define len length()
#define pushb push_back
#define fi first
#define se second

#define setpr fixed<<setprecision(15)
#define all(name) name.begin(),name.end()
#define rall(name) name.rbegin(),name.rend()
#define gsort(vbeg,vend) sort(vbeg,vend,greater<>())

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<class T>
inline void init(T& v) {
	for(auto &a: v) cin>>a;
}
template<class T, class U>
inline void init(vector<pair<T,U>>& v) {
	for(auto &a: v) cin>>a.first>>a.second;
}
template<class T, class N>
inline void init(T& v, N n) {
	v.resize(n);
	for(auto &a: v) cin>>a;
}
template<class T, class U, class N>
inline void init(vector<pair<T,U>>& v, N n) {
	v.resize(n);
	for(auto &a: v) cin>>a.first>>a.second;
}

inline void out() {
	cout<<endl;
}
template<class T, class... U>
inline void out(T a, U... alist) {
	cout<<a<<" ";
	out(forward<U>(alist)...);
}

template<class N>
void resiz(N n) {
	//empty
}
template<class N, class T, class... U>
void resiz(N n, T&& hd, U&&... tl) {
	hd.resize(n);
	resiz(n,forward<U>(tl)...);
}

bool isout(int h, int w, int H, int W) {
	return ( h<0 || h>=H || w<0 || w>=W );
}

ll binpow(ll a, ll ex, ll p) {
	ll result=1;
	while(ex>0) {
		if(ex&1) result=result*a%p;
		ex>>=1;
		a=a*a%p;
	}
	return result;
}


struct Combinatorics {
private:
	vector<ll> fact_, inv_, finv_;
	ll P;

	void pre_mod_comb(ll mx) {
		fact_.resize(mx+1);
		inv_.resize(mx+1);
		finv_.resize(mx+1);
		fact_[0] = fact_[1] = inv_[1] = finv_[0] = finv_[1] = 1LL;
		for(ll i=2LL; i<=mx; i++) {
			fact_[i] = fact_[i-1]*i%P;
			inv_[i] = P-inv_[P%i]*(P/i)%P;
			finv_[i] = finv_[i-1]*inv_[i]%P;
		}
	}

public:
	Combinatorics(ll mx, ll p) {
		P=p;
		pre_mod_comb(mx);
	}

	ll nPr(ll n, ll r) {
		assert(n>=0 && r>=0);
		if (r > n) return 0;
		return fact_[n]*finv_[n-r]%P;
	}

	ll nCr(ll n, ll r) {
		assert(n>=0 && r>=0);
		if (r > n) return 0;
		return fact_[n]*finv_[r]%P*finv_[n-r]%P;
	}

	ll nHr(ll n, ll r) {
		assert(n>=0 && r>=0);
		if(r == 0) return 1;
		if(n == 0) return 0;
		return nCr(n+r-1,n-1);
	}
};


int N,K;
intvec A;
ll ans;
Combinatorics comb(100000,MPRIME);

void input() {
	cin>>N>>K;
	init(A,N);
}

void solve() {
	sort(all(A));
	for(int i=0; i<N-K+1; i++) {
		ans = (ans + A[N-1-i]*comb.nCr(N-1-i,K-1))%MPRIME;
	}
	for(int i=0; i<N-K+1; i++) {
		ans = (MPRIME + ans + (-A[i])*comb.nCr(N-1-i,K-1)%MPRIME)%MPRIME;
	}
	cout<<ans<<endl;
}

int main() {
	cin.tie(nullptr);
	ios_base::sync_with_stdio(false);
	cout<<fixed<<setprecision(15);
	int t=1;
	while(t) {
		input();
		solve();
		t--;
	}
}


Measured memory_bytes: 6040