#pragma GCC optimize("Ofast")
#define _USE_MATH_DEFINES
#include "bits/stdc++.h"

using namespace std;

using u8 = uint8_t;
using u16 = uint16_t;
using u32 = uint32_t;
using u64 = uint64_t;
using i8 = int8_t;
using i32 = int32_t;
using i64 = int64_t;

constexpr char newl = '\n';
constexpr double eps = 1e-10;

#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define F0R(i,b) FOR(i,0,b)
#define RFO(i,a,b) for (int i = ((b)-1); i >=(a); i--)
#define RF0(i,b) RFO(i,0,b)
//#define fi first
//#define se second
#define debug(x) cout << #x << ": " << x << '\n';
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()

template<typename T1, typename T2> inline void chmin(T1& a, T2 b) { if (a > b) a = b; }
template<typename T1, typename T2> inline void chmax(T1& a, T2 b) { if (a < b) a = b; }
template<class T> void Print(vector<T> v) {
  F0R(i, v.size()) {
    cout << v[i] << ' ';
  }
  cout << newl;
}

#if 1

constexpr int MOD = 1e9 + 7; // 1e9 + 7

template<int MOD> struct Modint {
  long long val;
  constexpr Modint(long long v = 0) noexcept : val(v% MOD) {
    if (val < 0) val += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Modint operator - () const noexcept {
    return val ? MOD - val : 0;
  }
  constexpr Modint operator + (const Modint& r) const noexcept { return Modint(*this) += r; }
  constexpr Modint operator - (const Modint& r) const noexcept { return Modint(*this) -= r; }
  constexpr Modint operator * (const Modint& r) const noexcept { return Modint(*this) *= r; }
  constexpr Modint operator / (const Modint& r) const noexcept { return Modint(*this) /= r; }
  constexpr Modint& operator += (const Modint& r) noexcept {
    val += r.val;
    if (val >= MOD) val -= MOD;
    return *this;
  }
  constexpr Modint& operator -= (const Modint& r) noexcept {
    val -= r.val;
    if (val < 0) val += MOD;
    return *this;
  }
  constexpr Modint& operator *= (const Modint& r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Modint& operator /= (const Modint& r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b; swap(a, b);
      u -= t * v; swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0) val += MOD;
    return *this;
  }
  constexpr bool operator == (const Modint& r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator != (const Modint& r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream& operator << (ostream& os, const Modint<MOD>& x) noexcept {
    return os << x.val;
  }
  friend constexpr Modint<MOD> modpow(const Modint<MOD>& a, long long n) noexcept {
    if (n == 0) return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1) t = t * a;
    return t;
  }

  friend constexpr Modint<MOD> Ncr(Modint<MOD> a, int n) {
    Modint<MOD> p = 1, q = 1;

    while (n > 0) {
      p *= a;
      a -= 1;
      q *= n--;
    }

    return p / q;
  }
};
template<int MOD>
using mint = Modint<MOD>;
struct Factorial {

  int modpow(int a, u64 n) noexcept {
    if (n == 0) return 1;
    auto t = modpow(a, n / 2);
    t = (int)((i64)t * t % mod);
    if (n & 1) t = (int)((i64)t * a % mod);
    return t;
  }

  Factorial(int count, int mod, bool calcInverse = true) : mod(mod) {

    f.reserve(count + 1);
    if (calcInverse) fi.reserve(count + 1);

    f.push_back(1);
    if (calcInverse) fi.push_back(1);
    FOR(i, 1, count + 1) {
      f.push_back((int)(((i64)f[i - 1] * i) % mod));
      if (calcInverse) fi.push_back(modpow(f[i], mod - 2));
      //cout << i << " " << f[i] << " " << fi[i] << newl;
    }
  }

  int Ncr(int n, int r) {
    if (n < r) return 0;
    return (int)((i64)f[n] * fi[r] % mod * fi[n - r] % mod);
  }

  // without inverse array
  int Ncr2(int n, int r) {
    if (n < r) return 0;
    return Div(Div(f[n], f[r]), f[n - r]);
  }

  // 自分では何が起きているか分かっていないmod除算
  // - mod が素数じゃないと動かない模様
  // - 4 / 2 を mod100 で計算したら 4 になった
  int Div(int l, int r) noexcept {
    long long a = r, b = mod, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b; swap(a, b);
      u -= t * v; swap(u, v);
    }
    l = l * u % mod;
    if (l < 0) l += mod;
    return l;
  }
  // こちらも mod が素数じゃないと動かない様子
  // - 4 / 2 を mod100 で計算したら 76 になった
  /*
  int Div(int l, int r) noexcept {
    return l * modpow(r, mod - 2) % mod;
  }
  */

  int mod;

  // 階乗
  vector<int> f;

  // 階乗の逆元
  vector<int> fi;
};

// INSERT ABOVE HERE
signed main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  int N, K;
  cin >> N >> K;

  vector<i64> as(N);
  F0R(i, N) {
    cin >> as[i];
  }
  sort(rng(as));

  Factorial fc(2e5 + 1, MOD);

  mint<MOD> mins = 0;
  mint<MOD> maxs = 0;
  F0R(i, N) {
    
    mins += mint<MOD>(as[i]) * fc.Ncr(N - 1 - i, K - 1);

    maxs += mint<MOD>(as[N - i - 1]) * fc.Ncr(N - 1 - i, K - 1);
  }

  cout << (maxs - mins).val;

}
#endif


Measured memory_bytes: 2560