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

#define rep(i,n) for(ll i=0;i<(n);++i)
using ll = long long;
using pll = pair<ll,ll>;
constexpr ll INF = (1LL<<60);
constexpr ll MOD = (1e9+7);
//constexpr ll MOD = (998244353);
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}return 0;}
#if loc||debg||local||debug
void dump(){cerr<<endl;}
template<class T,class... Ts> void dump(T&& h, Ts&&... t){cerr<<h<<", ";dump(forward<Ts>(t)...);}
#else
template<class T,class... Ts> void dump(T&& h, Ts&&... t){}
#endif
template<class T> istream &operator>>(istream&is,vector<T>&v){for(auto &elemnt:v)is>>elemnt;return is;}
template<class T,class U> istream &operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<class T> ostream &operator<<(ostream& os,vector<T>const& v){for(auto const& vi:v)os<<vi<<" ";return os;}
template<class T>vector<T> vec(size_t a){return vector<T>(a);}
template<class T, class... Ts>auto vec(size_t a, Ts... ts){return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));}
template<class T>vector<T> _vec(size_t a,T v){return vector<T>(a,v);}
template<class T, class... Ts>auto _vec(size_t a, Ts... ts){return vector<decltype(_vec<T>(ts...))>(a, _vec<T>(ts...));}

template<long long MOD>
struct mod_int{
  using ll = long long;
  ll val;
  constexpr mod_int():val(0){}
  constexpr mod_int(ll x)noexcept:val(x>=0?x%MOD:(MOD-(-x)%MOD)%MOD){}
  constexpr ll value()const noexcept{return val;}
  constexpr mod_int operator+(mod_int const& rhs){
    return mod_int(*this)+=rhs;
  }
  constexpr mod_int operator-(mod_int const& rhs){
    return mod_int(*this)-=rhs;
  }
  constexpr mod_int operator*(mod_int const& rhs){
    return mod_int(*this)*=rhs;
  }
  constexpr mod_int operator/(mod_int const& rhs){
    return mod_int(*this)/=rhs;
  }
  constexpr mod_int &operator+=(mod_int const& rhs)noexcept{
    val += rhs.val;
    if(val>=MOD)val-=MOD;
    return *this;
  }
  constexpr mod_int &operator-=(mod_int const& rhs)noexcept{
    if(val<rhs.val)val+=MOD;
    val-=rhs.val;
    return *this;
  }
  constexpr mod_int &operator*=(mod_int const& rhs)noexcept{
    (val*=rhs.val)%=MOD;
    return *this;
  }
  constexpr mod_int &operator/=(mod_int rhs)noexcept{
    *this*=rhs.inverse();
    return *this;
  }
  constexpr bool operator==(mod_int const& rhs){
    return val==rhs.val;
  }
  constexpr bool operator!=(mod_int const& rhs){
    return !(val==rhs.val);
  }
  constexpr bool operator<(mod_int const& rhs){
    return val<rhs.val;
  }
  constexpr bool operator>(mod_int const& rhs){
    return val>rhs.val;
  }
  constexpr bool operator<=(mod_int const& rhs){
    return !(val>rhs.val);
  }
  constexpr bool operator>=(mod_int const& rhs){
    return !(val<rhs.val);
  }
  constexpr friend std::ostream &operator<<(std::ostream& os, mod_int const& mi){
    return os<<mi.val;
  }
  friend std::istream &operator>>(std::istream& is, mod_int & mi){
    ll t;is>>t;
    mi = mod_int<MOD>(t);
    return is;
  }
  constexpr mod_int inverse(){
    ll a=val,b=MOD,u=1,v=0,t;
    while(b>0){
      t=a/b;
      std::swap(a-=t*b,b);
      std::swap(u-=t*v,v);
    }
    return mod_int(u);
  }
  constexpr mod_int mpow(ll n)const{
    mod_int res(1),e(*this);
    while(n){
      if(n&1)res*=e;
      e*=e;
      n>>=1;
    }
    return res;
  }
};
using mint = mod_int<1000000007>;

constexpr ll COM_MAX = (ll)1e6;
vector<ll> fac(COM_MAX),finv(COM_MAX),inv(COM_MAX);
// 前処理
void comb_init(){
  fac[0]=fac[1]=1;
  finv[0]=finv[1]=1;
  inv[1]=1;
  for(int i=2;i<COM_MAX;++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;
  }
}
// 二項係数計算
ll comb(ll n,ll k){
  if(n<k)return 0;
  if(n<0||k<0)return 0;
  return fac[n]*(finv[k]*finv[n-k]%MOD)%MOD;
}
// 重複組み合わせ
ll nHk(ll n,ll k){
  return comb(n+k-1,n-1);
}


int main(){
  comb_init();

  ll n,k;
  cin>>n>>k;
  vector<ll> a(n);
  cin>>a;

  auto b = a;
  sort(b.begin(),b.end(),greater<>());

  mint ans = 0;
  rep(i,n){
    ans += mint(b[i])*comb(n-i-1,k-1);
  }
  reverse(b.begin(),b.end());
  rep(i,n){
    ans -= mint(b[i])*comb(n-i-1,k-1);
  }
  cout<<(ans)<<endl;

}

Measured memory_bytes: 26368