#include <iostream>
#include <set>
#include <vector>
#include <algorithm>
using namespace std;

typedef unsigned long long int ulint;
typedef long long int lint;

// Math
template< int mod >
struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if((x += p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int) (1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while(n > 0) {
      if(n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt< mod >(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

const ulint prime = 1000000000 + 7;

using modint = ModInt<prime>;

template< typename T >
struct Combination {
  vector< T > _fact, _rfact, _inv;

  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for(int i = 1; i <= sz; i++) _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for(int i = sz - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
    for(int i = 1; i <= sz; i++) _inv[i] = _rfact[i] * _fact[i - 1];
  }

  inline T fact(int k) const { return _fact[k]; }

  inline T rfact(int k) const { return _rfact[k]; }

  inline T inv(int k) const { return _inv[k]; }

  T P(int n, int r) const {
    if(r < 0 || n < r) return 0;
    return fact(n) * rfact(n - r);
  }

  T C(int p, int q) const {
    if(q < 0 || p < q) return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }

  T H(int n, int r) const {
    if(n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};


int main(){
  ulint N, K;
  cin >> N >> K;

  if (K == 1) {
    cout << 0 << endl;
    return 0;
  }

  vector<int> A;
  
  for (int i = 0; i < N; ++i) {
    int a;
    cin >> a;
    A.push_back(a);
  }
  
  sort(A.begin(), A.end());
  
  Combination<modint> cmb = Combination<modint>(N + 2);

  modint ret = 0;

  vector<modint> B(N);

  B[K - 2] = 1;
  
  for (int j = K - 1; j < N; ++j) {
    B[j] = B[j - 1] * j / (j - (K - 2));
  }

  vector<modint> fsum(N-1);
  
  fsum[0] = (modint) A[N - 1] - A[0];
  ret += fsum[0] * B[0];
  for (int d = 1; d < N - 1; ++d) {
    fsum[d] = fsum[d-1] + (modint) A[N - 1 - d] - (modint) A[d];
    
    //modint sums = 0;
    //
    //for (int i = 0; i < d + 1; ++i) {
    //  sums += (modint) A[N - 1 - i] - (modint) A[i];
    //}
    ret += fsum[d] * B[d];
  }
  
  //for (int i = 0; i < N; ++i) {
  //  for (int j = i + K - 1; j < N; ++j) {
  //    // cout << j - i - 1 << ", " << K-2 << endl;
  //    ret = ret +  B[j-i-1] * ((modint) (A[j] - A[i]));
  //  }
  //}
  
  cout << ret << endl;
 
}


Measured memory_bytes: 2680