#include <bits/stdc++.h>
#define LLI long long int
#define FOR(v, a, b) for(LLI v = (a); v < (b); ++v)
#define FORE(v, a, b) for(LLI v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(LLI v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define fst first
#define snd second
#define popcount __builtin_popcount
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <misc/C++/Debug.cpp>
#else
#define dump(...) ((void)0)
#endif

#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}
template <typename T, typename U> bool chmin(T &a, const U &b){return (a>b ? a=b, true : false);}
template <typename T, typename U> bool chmax(T &a, const U &b){return (a<b ? a=b, true : false);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}

struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);
    cerr << fixed << setprecision(12);
  }
}init;


template <std::uint32_t M> class ModInt{
public:
  std::uint64_t val;
  ModInt(): val(0){}
  ModInt(std::int64_t n){
    if(n >= M) val = n % M;
    else if(n < 0) val = n % M + M;
    else val = n;
  }
  
  inline constexpr ModInt operator+(const ModInt &a) const {return ModInt((val+a.val) % M);}
  inline constexpr ModInt operator-(const ModInt &a) const {return ModInt((val-a.val+M) % M);}
  inline constexpr ModInt operator*(const ModInt &a) const {return ModInt((val*a.val) % M);}
  inline constexpr ModInt operator/(const ModInt &a) const {return ModInt((val*a.inv().val) % M);}
  
  inline constexpr ModInt& operator=(const ModInt &a){val = a.val; return *this;}
  inline constexpr ModInt& operator+=(const ModInt &a){if((val += a.val) >= M) val -= M; return *this;}
  inline constexpr ModInt& operator-=(const ModInt &a){if(val < a.val) val += M; val -= a.val; return *this;}
  inline constexpr ModInt& operator*=(const ModInt &a){(val *= a.val) %= M; return *this;}
  inline constexpr ModInt& operator/=(const ModInt &a){(val *= a.inv().val) %= M; return *this;}

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

  inline constexpr ModInt& operator++(){*this += 1; return *this;}
  inline constexpr ModInt& operator--(){*this -= 1; return *this;}

  inline constexpr ModInt operator++(int){ModInt t = *this; *this += 1; return t;}
  inline constexpr ModInt operator--(int){ModInt t = *this; *this -= 1; return t;}

  inline constexpr static ModInt frac(std::int64_t a, std::int64_t b){
    return ModInt(a) / ModInt(b);
  }
  
  inline constexpr static ModInt power(std::int64_t n, std::int64_t p){
    if(p < 0) return power(n, -p).inv();
    
    ModInt ret = 1, e = n;
    for(; p; e *= e, p >>= 1) if(p&1) ret *= e;
    return ret;
  }

  inline constexpr ModInt power(std::int64_t p) const {return power(val,p);}
  
  inline constexpr ModInt inv() const {
    std::int64_t a = val, b = M, u = 1, v = 0;
    
    while(b){
      std::int64_t t = a/b;
      a -= t*b; std::swap(a,b);
      u -= t*v; std::swap(u,v);
    }
    u %= M;
    if(u < 0) u += M;
    
    return u;
  }
};

template <std::uint32_t M> ModInt<M> operator-(const ModInt<M> &a){return M-a.val;}

template <std::uint32_t M> ModInt<M> operator+(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)+b.val);}
template <std::uint32_t M> ModInt<M> operator-(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)-b.val);}
template <std::uint32_t M> ModInt<M> operator*(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)*b.val);}
template <std::uint32_t M> ModInt<M> operator/(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)/b.val);}

template <std::uint32_t M> std::istream& operator>>(std::istream &is, ModInt<M> &a){is >> a.val; return is;}
template <std::uint32_t M> std::ostream& operator<<(std::ostream &os, const ModInt<M> &a){ os << a.val; return os;}


/**
 * @attention 使用前にinit関数を呼び出す
 */
template <typename T> class Combinatorics{
public:
  static std::vector<T> facto;
  static std::vector<T> ifacto;

  static void init(int N){
    facto.assign(N+1, 1);
    ifacto.assign(N+1, 1);

    for(int i = 1; i <= N; ++i){
      facto[i] = facto[i-1] * i;
    }

    ifacto[N] = facto[N].inv();

    for(int i = N-1; i >= 0; --i){
      ifacto[i] = ifacto[i+1] * (i+1);
    }
  }

  static T f(int64_t i){
    assert(i < facto.size());
    return facto[i];
  }
  
  static T finv(int64_t i){
    assert(i < ifacto.size());
    return ifacto[i];
  }

  static T P(int64_t n, int64_t k);
  static T C(int64_t n, int64_t k);
  static T H(int64_t n, int64_t k);
  static T stirling_number(int64_t n, int64_t k);
  static T bell_number(int64_t n, int64_t k);
  static std::vector<T> bernoulli_number(int64_t n);
  static T catalan_number(int64_t n);
};

template <typename T> std::vector<T> Combinatorics<T>::facto = std::vector<T>();
template <typename T> std::vector<T> Combinatorics<T>::ifacto = std::vector<T>();

template <typename T> T Combinatorics<T>::P(int64_t n, int64_t k){
  if(n < k or n < 0 or k < 0) return 0;
  return f(n) * finv(n-k);
}

template <typename T> T Combinatorics<T>::C(int64_t n, int64_t k){
  if(n < k or n < 0 or k < 0) return 0;
  return P(n,k) * finv(k);
}

template <typename T> T Combinatorics<T>::H(int64_t n, int64_t k){
  if(n == 0 and k == 0) return 1;
  return C(n+k-1, k);
}


using mint = ModInt<1000000007>;
using C = Combinatorics<mint>;

template <typename T, typename Container=vector<T>> vector<pair<T,int64_t>> run_length_encode(const Container &v){
  vector<pair<T,int64_t>> ret;

  for(auto &x : v){
    if(ret.empty()) ret.push_back({x,1});
    else if(ret.back().fst == x) ++ret.back().snd;
    else ret.push_back({x,1});
  }

  return ret;
}




int main(){
  C::init(300000);
  
  int N, K;

  while(cin >> N >> K){
    vector<int> A(N); cin >> A;

    sort(ALL(A));

    mint ans = 0;

    {
      int s = 0;

      auto r = run_length_encode<int>(A);

      for(auto &p : r){

        //if(s < K) continue;        
        
        ans += (C::C(s+p.snd, K) - C::C(s, K)) * p.fst;

        s += p.snd;
      }
    }

    reverse(ALL(A));

    {
      int s = 0;

      auto r = run_length_encode<int>(A);

      for(auto &p : r){

        //if(s < K) continue;        
        
        ans -= (C::C(s+p.snd, K) - C::C(s, K)) * p.fst;

        s += p.snd;
      }
    }

    
    
    cout << ans << endl;
  }
  
  return 0;
}


Measured memory_bytes: 8768