#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cstdlib>
#include<cassert>

#include<cmath>
#include<functional>
#include<algorithm>

#include<vector>
#include<string>
#include<queue>
#include<stack>
#include<set>
#include<unordered_set>
#include<map>
#include<unordered_map>
#include<bitset>
#include<tuple>

#define TEST {IS_TEST=true;}
#define fi first
#define se second
#define pb(x) {push_back(x);}

using namespace std;
using ll = int_fast64_t;
using v_b = vector <bool>;
using v_ll = vector <ll>;
using str = string;
using v_str = vector <string>;
using p_ll = pair < ll,ll >;
using vv_b = vector < v_b >;
using vv_ll = vector < v_ll >;
using vp_ll = vector < p_ll >;
using vvv_ll = vector < vv_ll >;
using vvp_ll = vector < vp_ll >;

using ld = long double;
using v_ld = vector <ld>;
using vv_ld = vector<v_ld>;

bool IS_TEST=false;
ll ll_min64=1LL<<63;
ll ll_max64=~ll_min64;
ll ll_min32=1LL<<31;
ll ll_max32=~ll_min32;

ll MOD = 1000000007;

/*displaying functions for debug*/
template<class T> void show2(const T &x){cout << x;}
template<class T1,class T2> void show2(const pair<T1,T2> &x){
  cout << "{" << show2(x.first) << "," << show2(x.second) << "}";
}
template<class T> void show(const T &x){
  if (!IS_TEST) return;
  show2(x); cout <<endl;
}
template<class T> void v_show(const T &v, ll n=-1){
  if (!IS_TEST) return;
  auto itr=v.begin(); ll m=n;
  while(itr!=v.end() && m!=0 ){ show2(*itr); cout << " "; itr++; m--;}
  cout << endl;
}
template<class T> void vv_show(const T &v, ll n=-1){
  if (!IS_TEST) return;
  cout << "--------------------------------\n";
  auto itr=v.begin(); ll m=n;
  while(itr!=v.end() && m!=0 ){ v_show(*itr,n); itr++; m--;}
  cout << "--------------------------------" << endl;
}
/*--------------------------------*/

/*loading integers*/
void load(ll& x1){ cin >> x1; }
void load(ll& x1,ll& x2){ cin >> x1 >> x2; }
void load(ll& x1,ll& x2,ll& x3){ cin >> x1 >> x2 >> x3; }
void load(ll& x1,ll& x2,ll& x3,ll& x4){ cin >> x1 >> x2 >> x3 >> x4; }
void v_load(ll n, v_ll& v1, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head]; }
}
void v_load(ll n, v_ll& v1 ,v_ll& v2, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init); v2.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head] >> v2[i+head]; }
}
void v_load(ll n, v_ll& v1 ,v_ll& v2, v_ll& v3, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init); v2.assign(m,init); v3.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head] >> v2[i+head] >> v3[i+head]; }
}
void v_load(ll n, v_ll& v1 ,v_ll& v2, v_ll& v3, v_ll& v4, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init); v2.assign(m,init); v3.assign(m,init); v4.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head] >> v2[i+head] >> v3[i+head] >> v4[i+head]; }
}
/*--------------------------------*/

v_ll ll_mg(ll x1=ll_max64, ll x2=ll_max64, ll x3=ll_max64, ll x4=ll_max64){
    v_ll x{x1,x2,x3,x4};
    sort(x.begin(),x.end());
    return x;
}

template<class T> void ch_max(T& x,const T& y){ if (x>=y) return; x=y; }
template<class T> void ch_min(T& x,const T& y){ if (x<=y) return; x=y; }
template<class T,class S> void ch_max(T& x,S& xx,const T& y,const S& yy){
  if (x>=y) return; x=y; xx=yy;
}
template<class T,class S> void ch_min(T& x,S& xx,const T& y,const S& yy){
  if (x<=y) return; x=y; xx=yy;
}

template<class T> void quit(T x){cout << x << endl; exit(0);}

ll rem(ll x,ll y){ ll z=x%y; return z>=0?z:z+y; }


/*modint*/
class ModInt{
  public:
  static ll modulus;
  ll val;
  ModInt(ll n=0){ val=n%modulus; if (val<0) val+=modulus;}
  ModInt operator +(const ModInt& rhs)const{ return val+rhs.val;}
  ModInt operator -(const ModInt& rhs)const{ return val-rhs.val;}
  ModInt operator *(const ModInt& rhs)const{ return val*rhs.val;}
  ModInt& operator +=(const ModInt& rhs){val+=rhs.val; if (val>=modulus)val-=modulus; return *this;}
  ModInt& operator -=(const ModInt& rhs){val-=rhs.val; if (val<modulus)val+=modulus; return *this;}
  ModInt& operator *=(const ModInt& rhs){val*=rhs.val; val%=modulus; return *this;}
  ModInt pow(ll a)const{
    a%=(modulus-1); if (a<0)a+=(modulus-1);
    ModInt ret(1); ModInt vv(val);
    while (a!=0){ if (a&1) ret*=vv; a>>=1; vv*=vv;}
    return ret;
  }
  ModInt inv()const{return pow(-1);}
  ModInt operator /(const ModInt& rhs)const{ return *this*rhs.pow(-1);}
  ModInt& operator /=(const ModInt& rhs){ *this*=rhs.pow(-1); return *this;}
};
istream& operator >> (istream& istr, ModInt &rhs){istr >> (rhs.val); return istr;};
ostream& operator << (ostream& ostr, const ModInt &rhs){ostr << (rhs.val); return ostr;};
vector<ModInt> mod_nck(ll n){
  vector<ModInt> v(n+1,1);
  for (ll i=1;i<=n;i++) v[i]=v[i-1]*(n+1-i)/i;
  return v;
}
vector<ModInt> mod_npk(ll n){
  vector<ModInt> v(n+1,1);
  for (ll i=1;i<=n;i++) v[i]=v[i-1]*(n+1-i);
  return v;
}
vector<ModInt> mod_fact(ll n){
  vector<ModInt> v(n+1,1);
  for (ll i=1;i<=n;i++) v[i]=v[i-1]*i;
  return v;
}
vector<ModInt> mod_nck_2(ll n,ll k){
  vector<ModInt> v(n+1,1);
  for (ll i=k+1;i<=n;i++) v[i]=v[i-1]*i/(i-k);
  return v;
}

ll ModInt::modulus=1000000007;
//ll ModInt::modulus=998244353;


ll N,K,ans;
v_ll A;
int main(){
  load(N,K);
  v_load(N,A);
  sort(A.begin(),A.end());
  ModInt x;
  vector<ModInt> v(N,0);
  v[K-1]=1;
  for(ll i=K; i<N;i++) {
    v[i]=v[i-1]*i;
    v[i]/=(i-K+1);
  }

  v_show(v);
  for(ll i=0; i<N;i++){
    ModInt a(v[N-1-i]*ModInt(A[i]));
    ModInt a2(v[i]*ModInt(A[i]));
    x-=a;
    x+=a2;
  }
  cout << x.val%MOD << endl;
}




Measured memory_bytes: 2424