#include <bits/stdc++.h>
#define debug(x) cerr << #x << ": " << x << endl
#define debugArray(x, n)                           \
  for (long long hoge = 0; (hoge) < (n); ++(hoge)) \
  cerr << #x << "[" << hoge << "]: " << x[hoge] << endl
using namespace std;

template <int mod>
struct ModInt {
  int x;
  ModInt() : x(0) {}
  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % 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) { return *this *= p.inverse(); }
  ModInt operator-() const { return ModInt() - *this; }
  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) t = a / b, swap(a -= t * b, b), swap(u -= t * v, v);
    return ModInt(u);
  }
  ModInt pow(int64_t e) const {
    ModInt ret(1);
    for (ModInt b = *this; e; e >>= 1, b *= b)
      if (e & 1) ret *= b;
    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 modulo() { return mod; }
};

template <class Modint>
struct Combination {
  static vector<Modint> _fact, _finv, _inv;
  static void init(int sz) {
    int n = min(2 * sz, Modint::modulo() - 1);
    _fact.resize(n + 1), _finv.resize(n + 1), _inv.resize(n + 1);
    for (int i = 0; i <= n; ++i) _fact[i] = i ? Modint(i) * _fact[i - 1] : 1;
    _finv[n] = _fact[n].inverse();
    for (int i = n; i; --i) _finv[i - 1] = Modint(i) * _finv[i];
    for (int i = 1; i <= n; ++i) _inv[i] = _finv[i] * _fact[i - 1];
  }
  static Modint inverse(int n) { return _inv[n]; }
  static Modint fact(int n, bool inv = 0) { return inv ? _finv[n] : _fact[n]; }
  static Modint nPr(int n, int r) {
    if (n < r || r < 0) return Modint(0);
    return _fact[n] * _finv[n - r];
  }
  static Modint nCr(int n, int r) { return nPr(n, r) * _finv[r]; }
  static Modint nHr(int n, int r) { return !r ? Modint(1) : nCr(n + r - 1, r); }
  static size_t size() { return _inv.size(); }
};
template <class Modint>
vector<Modint> Combination<Modint>::_fact;
template <class Modint>
vector<Modint> Combination<Modint>::_finv;
template <class Modint>
vector<Modint> Combination<Modint>::_inv;

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  using Mint = ModInt<int(1e9 + 7)>;
  using Comb = Combination<Mint>;
  int N, K;
  cin >> N >> K;
  Comb::init(N + K);
  long long A[N];
  for (int i = 0; i < N; i++) cin >> A[i];
  sort(A, A + N);
  Mint ans = 0;
  for (int i = 0; i < N; i++) {
    int j = i;
    while (j < N && A[i] == A[j]) j++;
    ans += (Comb::nCr(j, K) - Comb::nCr(i, K)) * Mint(A[i]);
    ans -= (Comb::nCr(N - i, K) - Comb::nCr(N - j, K)) * Mint(A[i]);
    i = j - 1;
  }
  cout << ans << endl;
  return 0;
}


Measured memory_bytes: 7412