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

template <unsigned int mod> class modint {
private:
  unsigned int v;
  static unsigned int norm(const unsigned int &x) { return x < mod ? x : x - mod; }
  static modint make(const unsigned int &x) { modint m; return m.v = x, m; }
  static modint inv(const modint &x) { return make(inverse(x.v, mod)); }
  static unsigned int inverse(int a, int m) {
    int u[] = {a, 1, 0}, v[] = {m, 0, 1}, t;
    while (*v) {
      t = *u / *v;
      swap(u[0] -= t * v[0], v[0]), swap(u[1] -= t * v[1], v[1]), swap(u[2] -= t * v[2], v[2]);
    }
    return (u[1] % m + m) % m;
  }
public:
  modint() : v{0}{}
  modint(const long val) : v{norm(val % mod + mod)} {}
  modint(const modint<mod>& n) : v{n()} {}
  explicit operator bool() const noexcept { return v != 0; }
  bool operator!() const noexcept { return !static_cast<bool>(*this); }
  modint& operator=(const modint& n) { return v = n(), (*this); }
  modint& operator=(const long val) { return v = norm(val % mod + mod), (*this); }
  modint operator+() const { return *this; }
  modint operator-() const { return v == 0 ? make(0) : make(mod - v); }
  modint operator+(const modint& val) const { return make(norm(v + val())); }
  modint operator-(const modint& val) const { return make(norm(v + mod - val())); }
  modint operator*(const modint& val) const { return make((long) v * val() % mod); }
  modint operator/(const modint& val) const { return *this * inv(val); }
  modint& operator+=(const modint& val) { return *this = *this + val; }
  modint& operator-=(const modint& val) { return *this = *this - val; }
  modint& operator*=(const modint& val) { return *this = *this * val; }
  modint& operator/=(const modint& val) { return *this = *this / val; }
  modint operator+(const long val) const { return modint{v + val}; }
  modint operator-(const long val) const { return modint{v - val}; }
  modint operator*(const long val) const { return modint{(long) (v * (val % mod))}; }
  modint operator/(const long val) const { return modint{(long) v * inv(val)}; }
  modint& operator+=(const long val) { return *this = *this + val; }
  modint& operator-=(const long val) { return *this = *this - val; }
  modint& operator*=(const long val) { return *this = *this * val; }
  modint& operator/=(const long val) { return *this = *this / val; }
  bool operator==(const modint& val) const { return v == val.v; }
  bool operator!=(const modint& val) const { return !(*this == val); }
  bool operator==(const long val) const { return v == norm(val % mod + mod); }
  bool operator!=(const long val) const { return !(*this == val); }
  unsigned int operator()() const { return v; }
  friend modint operator+(const long val, const modint& n) { return n + val; }
  friend modint operator-(const long val, const modint& n) { return modint{val - n()}; }
  friend modint operator*(const long val, const modint& n) { return n * val; }
  friend modint operator/(const long val, const modint& n) { return modint{val} / n; }
  friend bool operator==(const long val, const modint& n) { return n == val; }
  friend bool operator!=(const long val, const modint& n) { return !(val == n); }
  friend istream& operator>>(istream& is, modint& n) {
    unsigned int v;
    return is >> v, n = v, is;
  }
  friend ostream& operator<<(ostream& os, const modint& n) { return (os << n()); }
  friend modint mod_pow(modint x, long n) {
    modint ans = 1;
    while (n) {
      if (n & 1) ans *= x;
      x *= x, n >>= 1;
    }
    return ans;
  }
};

constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int MAX = 1e5;
using mint = modint<MOD>;

vector<long> fact(MAX), finv(MAX), inve(MAX);

void init() {
  fact.at(0) = fact.at(1) = 1;
  finv.at(0) = finv.at(1) = 1;
  inve.at(1) = 1;
  for (int i = 2; i < MAX; i++) {
    fact.at(i) = fact.at(i - 1) * i % MOD;
    inve.at(i) = MOD - inve.at(MOD % i) * (MOD / i) % MOD;
    finv.at(i) = finv.at(i - 1) * inve.at(i) % MOD;
  }
}

long com(long n, long r) {
  if (n < r || n < 0 || r < 0) return 0;
  return fact.at(n) * (finv.at(r) * finv.at(n - r) % MOD) % MOD;
}

int main() {
  init();
  long N, K;
  cin >> N >> K;
  vector<long> A(N);
  for (int i = 0; i < N; i++) cin >> A.at(i);
  sort(A.begin(), A.end());
  mint ans = 0;
  for (int i = 0; i < N; i++) {
    ans += com(i, K - 1) * A.at(i);
    ans -= com(N - i - 1, K - 1) * A.at(i);
  }
  cout << ans << "\n";
}


Measured memory_bytes: 6364