#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <deque>
#include <fstream>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <vector>
using namespace std;
using ll = long long;
#define fst first
#define snd second

/* clang-format off */
template <class T, size_t D> struct _vec { using type = vector<typename _vec<T, D - 1>::type>; };
template <class T> struct _vec<T, 0> { using type = T; };
template <class T, size_t D> using vec = typename _vec<T, D>::type;
template <class T> vector<T> make_v(size_t size, const T& init) { return vector<T>(size, init); }
template <class... Ts> auto make_v(size_t size, Ts... rest) { return vector<decltype(make_v(rest...))>(size, make_v(rest...)); }
template <class T> inline void chmin(T &a, const T& b) { if (b < a) a = b; }
template <class T> inline void chmax(T &a, const T& b) { if (b > a) a = b; }
/* clang-format on */

template <std::uint_fast64_t Modulus>
class modint {
  using u64 = std::uint_fast64_t;

 public:
  u64 a;

  constexpr modint(const u64 x = 0) noexcept
      : a(x % Modulus) {
  }
  constexpr u64 &value() noexcept {
    return a;
  }
  constexpr const u64 &value() const noexcept {
    return a;
  }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(const modint rhs) noexcept {
    return *this *= ~rhs;
  }
  constexpr modint operator^(u64 exp) const noexcept {
    modint v = 1, x = *this;
    while (exp) {
      if (exp & 1) {
        v *= x;
      }
      x *= x;
      exp >>= 1;
    }
    return v;
  }
  constexpr modint operator~() const noexcept {
    return *this ^ (Modulus - 2);
  }
};

const int MOD = 1000000007;
using mint = modint<1000000007>;

vector<mint> fact, factInv;
void initFact(int N) {
  fact.resize(N + 1);
  factInv.resize(N + 1);
  fact[0] = 1;
  for (int i = 1; i <= N; i++) fact[i] = fact[i - 1] * i;
  factInv[N] = ~fact[N];
  for (int i = N - 1; i >= 0; i--) factInv[i] = factInv[i + 1] * (i + 1);
}

mint choose(int n, int k) {
  if (n < 0 || k < 0 || k > n) {
    return 0;
  } else {
    return fact[n] * factInv[k] * factInv[n - k];
  }
}

int main() {
#ifdef DEBUG
  ifstream ifs("in.txt");
  cin.rdbuf(ifs.rdbuf());
#endif
  int N, K;
  while (cin >> N >> K) {
    initFact(N + 10);
    vector<ll> A(N);
    for (auto& x : A) cin >> x;
    sort(A.begin(), A.end());
    mint res = 0;
    for (int L = K; L <= N; L++) res += choose(L - 1, K - 1) * (MOD + A[L - 1]);
    reverse(A.begin(), A.end());
    for (int L = K; L <= N; L++) res -= choose(L - 1, K - 1) * (MOD + A[L - 1]);
    cout << res.value() << endl;
  }
  return 0;
}


Measured memory_bytes: 2560