#pragma GCC optimize("Ofast")
#include <cstdio>
#include <cstdlib>
#include <string>
#include <algorithm>
#include <iostream>
#include <queue>
#include <vector>
#include <bitset>
#include <cmath>
#include <limits>
#include <iostream>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <iomanip>
#include <functional>
#include <complex>

using namespace std;

using ll=long long int;
using pii=pair<int,int>;
using pil=pair<int,ll>;
using pli=pair<ll,int>;
using pll=pair<ll,ll>;
using psi=pair<string,int>;
using pis=pair<int,string>;
using psl=pair<string,ll>;
using pls=pair<ll,string>;
using pss=pair<string,string>;



template<typename T> using vc=vector<T>;
template<typename T> using vvc=vector<vector<T>>;
template<typename T> using vvvc=vector<vector<vector<T>>>;
template<typename T> using vvvvc=vector<vvvc<T>>;
template<typename T> using vvvvvc=vector<vvvvc<T>>;

template<class T,class U>inline constexpr bool chmin(T&a,const U b){if(a<=b)return false;a=b;return true;}
template<class T,class U>inline constexpr bool chmax(T&a,const U b){if(a>=b)return false;a=b;return true;}
#define bit(n,k) ( (n>>k)&1 )

inline void bin101(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr); 
    cout<<fixed<<setprecision(20);
}

//デバッグ
template<typename T>
inline void vout(const vector<T> &v){
    cout<<"\nstart\n";
    const int sz=v.size();
    for(int i=0;i<sz;i++){
        cout<<i<<" "<<v[i]<<'\n';
    }
    cout<<"end\n"<<endl;
}
//デバッグ
template<typename T>
inline void vout(const vvc<T> &v){
    cout<<"\nstart\n";
    const int sz=v.size();
    for(int i=0;i<sz;i++){
        int ssz=v[i].size();
        for(int j=0;j<ssz;j++){
            cout<<i<<" "<<j<<" "<<v[i][j]<<'\n';
        }
    }
    cout<<"end\n"<<endl;
}
//デバッグ
template<typename T>
inline void vout(const vvvc<T> &v){
    cout<<"\nstart\n";
    const int sz=v.size();
    for(int i=0;i<sz;i++){
        int ssz=v[i].size();
        for(int j=0;j<ssz;j++){
            int sssz=v[i][j].size();
            for(int k=0;k<sssz;k++){
                cout<<i<<" "<<j<<" "<<k<<" "<<v[i][j][k]<<'\n';
            }
        }
    }
    cout<<"end\n"<<endl;
}


//pair cout
template<typename T, typename U>
inline ostream &operator<<(ostream &os,const pair<T,U> &p) {
  os<<p.first<<" "<<p.second;
  return os;
}

//pair cin
template<typename T, typename U>
inline istream &operator>>(istream &is,pair<T,U> &p) {
  is>>p.first>>p.second;
  return is;
}

//ソート
template<typename T>
inline void vsort(vector<T> &v){
    sort(v.begin(),v.end());
}

//逆順ソート
template<typename T>
inline void rvsort(vector<T> &v){
  sort(v.rbegin(),v.rend());
}

//要素数a 初期値x
template<typename T>
inline vector<T> vmake(int a,T x){
  return vector<T>(a,x);
}

//data[a][b] 初期値x
template<typename T>
inline vector<vector<T>> vmake(int a,int b,T x){
  return vector<vector<T>>(a, vector<T>(b,x));
}

//data[a][b][c] 初期値x
template<typename T>
inline vector<vector<vector<T>>> vmake(int a,int b,int c,T x){
  return vector<vector<vector<T>>> (a, vector<vector<T>>(b, vector<T>(c, x)));
}

//data[a][b][c][d]　初期値x
template<typename T>
inline vector<vector<vector<vector<T>>>> vmake(int a,int b,int c,int d,T x){
  return vector<vector<vector<vector<T>>>> (a,vvvc<T>(b,vvc<T>(c,vc<T>(d,x))));
}

//data[a][b][c][d][e] 初期値x
template<typename T>
inline vvvvvc<T> vmake(int a,int b,int c,int d,int e,T x){
  return vvvvvc<T> (a,vvvvc<T>(b,vvvc<T>(c,vvc<T>(d,vc<T>(e,x)))));
}

//1ビットの数を返す
inline int popcount(int x){
  return __builtin_popcount(x);
}
//1ビットの数を返す
inline int popcount(ll x){
  return __builtin_popcountll(x);
}

//queのfront() pop()
template<typename T>
inline T pop(queue<T> &que){
    T x=que.front();
    que.pop();
    return x;
}

//priority_que top() pop()
template<typename T>
inline T pop(priority_queue<T> &que){
  T x=que.top();
  que.pop();
  return x;
}

//stack top() pop()
template<typename T>
inline T pop(stack<T> &st){
  T x=st.top();
  st.pop();
  return x;
}

#define intsz(x) ( (int)x.size()  )

/*
満たすものの個数を返す
mode:0 x未満
mode:1 x以下の数
mode:2 x以上の数
mode:3 x超
mode:4 x
*/
template<typename T>
inline int count_bound(vector<T> &v,T x,int mode){
    switch(mode){
        case 0:
            return lower_bound(v.begin(),v.end(),x)-v.begin();
        case 1:
            return upper_bound(v.begin(),v.end(),x)-v.begin();
        case 2:
            return v.end()-lower_bound(v.begin(),v.end(),x);
        case 3:
            return v.end()-upper_bound(v.begin(),v.end(),x);
        case 4:
            return upper_bound(v.begin(),v.end(),x)-lower_bound(v.begin(),v.end(),x);
    }
}

/*
mode:0 xより小さい数で最大の数
mode:1 x以下の数で最大の数
mode:2 x以上の数で最小の数
mode:3 xより大きい数で最小の数
*/
template<typename T>
inline T value_bound(vector<T> &v,T x,int mode){
    switch(mode){
        case 0:
            return *(--lower_bound(v.begin(),v.end(),x));
        case 1:
            return *(--upper_bound(v.begin(),v.end(),x));
        case 2:
            return *lower_bound(v.begin(),v.end(),x);
        case 3:
            return *upper_bound(v.begin(),v.end(),x);
    }
}



constexpr int MAX=1<<30;
constexpr ll INF=1LL<<62;
constexpr ll MOD=1e9+7;
int dx[]={1,-1,0,0},dy[]={0,0,-1,1};

//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

struct ModInt{
    using u64=uint_fast64_t;
    u64 a;
    constexpr ModInt() :a(0){}
    constexpr ModInt(ll x) :a((x>=0)?(x%MOD):(x%MOD+MOD) ) {}

    inline constexpr ModInt operator+(const ModInt rhs)const noexcept{
        return ModInt(*this)+=rhs;
    }
    inline constexpr ModInt operator-(const ModInt rhs)const noexcept{
        return ModInt(*this)-=rhs;
    }
    inline constexpr ModInt operator*(const ModInt rhs)const noexcept{
        return ModInt(*this)*=rhs;
    }
    inline constexpr ModInt operator/(const ModInt rhs)const noexcept{
        return ModInt(*this)/=rhs;
    }
    inline constexpr ModInt operator+(const ll rhs) const noexcept{
        return ModInt(*this)+=rhs;
    }
    inline constexpr ModInt operator-(const ll rhs)const noexcept{
        return ModInt(*this)-=rhs;
    }
    inline constexpr ModInt operator*(const ll rhs)const noexcept{
        return ModInt(*this)*=rhs;
    }
    inline constexpr ModInt operator/(const ll rhs)const noexcept{
        return ModInt(*this)/=rhs;
    }

    inline constexpr ModInt &operator+=(const ModInt rhs)noexcept{
        a+=rhs.a;
        if(a>=MOD) a-=MOD;
        return *this;
    }
    inline constexpr ModInt &operator-=(const ModInt rhs)noexcept{
        if(rhs.a>a) a+=MOD;
        a-=rhs.a;
        return *this;
    }
    inline constexpr ModInt &operator*=(const ModInt rhs)noexcept{
        a=(a*rhs.a)%MOD;
        return *this;
    }
    inline constexpr ModInt &operator/=(ModInt rhs)noexcept{
        a=(a*rhs.inverse().a)%MOD;
        return *this;
    }
    inline constexpr ModInt &operator+=(const ll rhs)noexcept{
        a+=rhs;
        if(a>=MOD) a-=MOD;
        return *this;
    }
    inline constexpr ModInt &operator-=(const ll rhs)noexcept{
        if(rhs>a) a+=MOD;
        a-=rhs;
        return *this;
    }
    inline constexpr ModInt &operator*=(const ll rhs)noexcept{
        a=(a*rhs)%MOD;
        return *this;
    }
    inline constexpr ModInt &operator/=(ll rhs)noexcept{
        a=(a*ModInt(rhs).inverse().a)%MOD;
        return *this;
    }

    inline constexpr ModInt operator=(ll x)noexcept{
        x%=MOD;
        if(x<0) x+=MOD;
        a=x;
        return *this;
    }

    inline constexpr bool operator==(const ModInt p)const noexcept{
        return a==p.a;
    }

    inline constexpr bool operator!=(const ModInt p)const noexcept{
        return a!=p.a;
    }

    inline constexpr ModInt pow(ll N) const noexcept{
        ModInt ans(1LL),p(a);
        while(N>0){
            if(bit(N,0)){
                ans*=p;
            }
            p*=p;
            N>>=1;
        }
        return ans;
    }
    inline constexpr ModInt inverse() const noexcept{
        return pow(MOD-2);
    }

};
inline constexpr ModInt operator+(const ll &a,const ModInt &b)noexcept{
    return ModInt(a)+=b;
}
inline constexpr ModInt operator-(const ll &a,const ModInt &b)noexcept{
    return ModInt(a)-=b;
}
inline constexpr ModInt operator*(const ll &a,const ModInt &b)noexcept{
    return ModInt(a)*=b;
}
inline constexpr ModInt operator/(const ll &a,const ModInt &b)noexcept{
    return ModInt(a)/=b;
}
//cout
inline ostream &operator<<(ostream &os,const ModInt &p) {
    return os<<p.a;
}

//cin
inline istream &operator>>(istream &is,ModInt &p) {
    ll t;
    is>>t;
    p=t;
    return is;
}



inline constexpr ModInt Mr(ll p){
    p%=MOD;
    if(p<0) p+=MOD;
    return ModInt(p);
}

struct Binominal{
    vector<ModInt> fac,finv,inv; //fac[n]:n! finv:(n!)の逆元
    int sz;
    Binominal(int n=10) :sz(0){
        if(n==0) n=10;
        init(n);
    }
    inline void init(int n){
        fac.resize(n+1,1);
        finv.resize(n+1,1);
        inv.resize(n+1,1);
        for(int i=2;i<=n;i++){
            fac[i]=fac[i-1]*i;
            inv[i]=MOD-inv[MOD%i]*(MOD/i);
            finv[i]=finv[i-1]*inv[i];
        }
        sz=n;
    }
    //nCk(n,k<=N) をO(1)で求める
    inline ModInt com(int n,int k){
        if(n<k) return ModInt(0);
        if(n<0 || k<0) return ModInt(0);
        if(n>sz) init(n);
        return fac[n]*finv[k]*finv[n-k];
    }
    //nCk(k<=N) をO(k)　で求める 
    inline ModInt rcom(ll n,int k){
        if(n<0 || k<0 || n<k) return ModInt(0);
        if(k>sz) init(k);
        ModInt ret(1);
        for(int i=0;i<k;i++){
            ret*=(n-i);
        }
        ret*=finv[k];
        return ret;
    }

    //重複組み合わせ n種類のものから重複を許してk個を選ぶ
    //〇がn個,|がk個
    inline ModInt h(int n,int k){
        return com(n+k-1,k);
    }
};


signed main(){
    //bin101();

    int n,k;
    cin>>n>>k;
    vector<ll> A(n);
    for(int i=0;i<n;i++) cin>>A[i];
    vsort(A);
    Binominal b;
    ModInt ans(0);
    for(int i=0;i<n;i++){
      ans+=A[i]*(b.com(i,k-1)-b.com(n-i-1,k-1));
    }
    cout<<ans<<endl;
}

Measured memory_bytes: 3328