#include "bits/stdc++.h"
#include <random>
#define REP(i, n) for (ll i = 0; i < ll(n); ++i)
#define RREP(i, n) for (ll i = ll(n) - 1; i >= 0; --i)
#define FOR(i, m, n) for (ll i = m; i < ll(n); ++i)
#define RFOR(i, m, n) for (ll i = ll(n) - 1; i >= ll(m); --i)
#define ALL(v) (v).begin(), (v).end()
#define UNIQUE(v) v.erase(unique(ALL(v)), v.end());
#define INF 1000000001ll
#define MOD 1000000007ll
#define EPS 1e-9

constexpr int dx[8] = { 1, 1, 0, -1, -1, -1, 0, 1 };
constexpr int dy[8] = { 0, 1, 1, 1, 0, -1, -1, -1 };

using namespace std;

using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

template <class T> bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

#include "bits/stdc++.h"
template <int mod, bool isPrime = false> class Mint {
  using ll = long long;
  bool dirty;
  ll value;

public:
  static std::vector<Mint> fact;
  static std::vector<Mint> rfact;
  constexpr Mint() : value(0), dirty(false) {};
  constexpr Mint(ll x) : value(x), dirty(true) {
    value %= mod;
    if (value < 0)
      value += mod;
  }
  constexpr Mint(const Mint& other) : value(other.value), dirty(other.dirty) {}

  ll& get() {
    return value;
  }
  ll get() const {
    return value;
  }

  constexpr Mint pow(ll n) {
    Mint ret(1), tmp(value);
    while (n) {
      if (n & 1)
        ret *= tmp;
      n >>= 1;
      tmp *= tmp;
    }
    return ret;
  }
  constexpr bool initialized() const {
    return dirty;
  }
  constexpr bool operator==(const Mint& a) const {
    return value == a.value;
  }
  constexpr bool operator!=(const Mint& a) const {
    return value != a.value;
  }
  constexpr Mint& operator=(const Mint& a) {
    value = a.value;
    dirty = true;
    return *this;
  }
  constexpr Mint& operator+=(const Mint& a) {
    if ((value += a.value) >= mod)
      value -= mod;
    dirty = true;
    return *this;
  }
  constexpr Mint& operator-=(const Mint& a) {
    if ((value += mod - a.value) >= mod)
      value -= mod;
    dirty = true;
    return *this;
  }
  constexpr Mint& operator*=(const Mint& a) {
    value = value * a.value % mod;
    dirty = true;
    return *this;
  }

  constexpr Mint& operator/=(const Mint& a) {
    initialized = true;
    return *this *= inv(a);
  }
  constexpr Mint operator+(const Mint& a) const {
    return Mint(*this) += a;
  }
  constexpr Mint operator-(const Mint& a) const {
    return Mint(*this) -= a;
  }
  constexpr Mint operator*(const Mint& a) const {
    return Mint(*this) *= a;
  }
  constexpr Mint operator/(const Mint& a) const {
    return Mint(*this) /= a;
  }
  static constexpr void init_comb(int n) {
    fact.resize(n + 1);
    rfact.resize(n + 1);
    fact[0] = 1;
    for (int i = 0; i < n; ++i) {
      fact[i + 1] = fact[i] * (i + 1);
    }
    rfact[n] = inv(fact[n]);
    for (int i = n - 1; i >= 0; --i) {
      rfact[i] = rfact[i + 1] * (i + 1);
    }
  }
  static constexpr Mint comb(int a, int b) {
    if (a < 0 || b < 0 || a >= fact.size() || b >= fact.size() || a < b)
      return Mint(0);
    return fact[a] * rfact[b] * rfact[a - b];
  }
};

template <int mod, bool isPrime>
std::ostream& operator<<(std::ostream& os, const Mint<mod, isPrime>& m) {
  os << m.get();
  return os;
}
template <int mod, bool isPrime>
std::istream& operator>>(std::istream& is, Mint<mod, isPrime>& m) {
  is >> m.get();
  return is;
}

template <int mod> constexpr Mint<mod, true> inv(const Mint<mod, true>& m) {
  long long a = m.get(), b = mod, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    std::swap(a, b);
    u -= t * v;
    std::swap(u, v);
  }
  return Mint<mod, true>(u);
}

template <int mod, bool isPrime>
constexpr Mint<mod, isPrime> operator+(long long lhs,
  const Mint<mod, isPrime>& a) {
  return Mint<mod, isPrime>(lhs + a.get());
}

template <int mod, bool isPrime>
constexpr Mint<mod, isPrime> operator-(long long lhs,
  const Mint<mod, isPrime>& a) {
  return Mint<mod, isPrime>(lhs - a.get());
}

template <int mod, bool isPrime>
constexpr Mint<mod, isPrime> operator*(long long lhs,
  const Mint<mod, isPrime>& a) {
  return Mint<mod, isPrime>(lhs % mod * a.get());
}

template <int mod>
constexpr Mint<mod, true> operator/(long long lhs, const Mint<mod, true>& a) {
  return Mint<mod, true>(lhs % mod * inv(a));
}
template <int mod, bool isPrime>
std::vector<Mint<mod, isPrime>>
Mint<mod, isPrime>::fact = std::vector<Mint<mod, isPrime>>();

template <int mod, bool isPrime>
std::vector<Mint<mod, isPrime>>
Mint<mod, isPrime>::rfact = std::vector<Mint<mod, isPrime>>();

constexpr int default_mod = 1000000007;
using mint = Mint<default_mod, true>;


int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  vi v(n);
  REP(i, n)cin >> v[i];
  vector<mint> fact(n + 1), rfact(n + 1);
  fact[0] = 1;
  REP(i, n)fact[i + 1] = fact[i] * (i + 1);
  rfact[n] = inv(fact[n]);
  RREP(i, n)rfact[i] = rfact[i + 1] * (i + 1);
  auto comb = [&](int a, int b) {
    if (a < 0 || b < 0 || a - b < 0)return mint(0);
    return fact[a] * rfact[b] * rfact[a - b];
  };
  sort(ALL(v));
  mint ans = 0;
  REP(i, n - 1) {
    ans += (v[i + 1] - v[i]) * (comb(n, k) - comb(i+1,k) - comb(n-i-1,k));
  }
  cout << ans << endl;
}


Measured memory_bytes: 3840