#include <bits/stdc++.h>
#define rep(i, n) for (lli i = 0; i < (n); i++)
#define rrep(i, n) for (lli i = (n)-1; i >= 0; i--)
using namespace std;
using lli = long long int;
template <signed M, unsigned T>
struct mod_int {
    constexpr static signed MODULO = M;
    constexpr static unsigned TABLE_SIZE = T;

    signed x;

    mod_int() : x(0) {}

    mod_int(long long y) : x(static_cast<signed>(y >= 0 ? y % MODULO : MODULO - (-y) % MODULO)) {}

    mod_int(int y) : x(y >= 0 ? y % MODULO : MODULO - (-y) % MODULO) {}

    mod_int& operator+=(const mod_int& rhs)
    {
        if ((x += rhs.x) >= MODULO)
            x -= MODULO;
        return *this;
    }

    mod_int& operator-=(const mod_int& rhs)
    {
        if ((x += MODULO - rhs.x) >= MODULO)
            x -= MODULO;
        return *this;
    }

    mod_int& operator*=(const mod_int& rhs)
    {
        x = static_cast<signed>(1LL * x * rhs.x % MODULO);
        return *this;
    }

    mod_int& operator/=(const mod_int& rhs)
    {
        x = static_cast<signed>((1LL * x * rhs.inv().x) % MODULO);
        return *this;
    }

    mod_int operator-() const { return mod_int(-x); }

    mod_int operator+(const mod_int& rhs) const { return mod_int(*this) += rhs; }

    mod_int operator-(const mod_int& rhs) const { return mod_int(*this) -= rhs; }

    mod_int operator*(const mod_int& rhs) const { return mod_int(*this) *= rhs; }

    mod_int operator/(const mod_int& rhs) const { return mod_int(*this) /= rhs; }

    bool operator<(const mod_int& rhs) const { return x < rhs.x; }

    mod_int inv() const
    {
        assert(x != 0);
        if (x <= static_cast<signed>(TABLE_SIZE)) {
            if (_inv[1].x == 0)
                prepare();
            return _inv[x];
        } else {
            signed a = x, b = MODULO, u = 1, v = 0, t;
            while (b) {
                t = a / b;
                a -= t * b;
                std::swap(a, b);
                u -= t * v;
                std::swap(u, v);
            }
            return mod_int(u);
        }
    }

    mod_int pow(long long t) const
    {
        assert(!(x == 0 && t == 0));
        mod_int e = *this, res = mod_int(1);
        for (; t; e *= e, t >>= 1)
            if (t & 1)
                res *= e;
        return res;
    }

    mod_int fact()
    {
        if (_fact[0].x == 0)
            prepare();
        return _fact[x];
    }

    mod_int inv_fact()
    {
        if (_fact[0].x == 0)
            prepare();
        return _inv_fact[x];
    }

    mod_int choose(mod_int y)
    {
        assert(y.x <= x);
        return this->fact() * y.inv_fact() * mod_int(x - y.x).inv_fact();
    }

    static mod_int _inv[TABLE_SIZE + 1];

    static mod_int _fact[TABLE_SIZE + 1];

    static mod_int _inv_fact[TABLE_SIZE + 1];

    static void prepare()
    {
        _inv[1] = 1;
        for (int i = 2; i <= (int)TABLE_SIZE; ++i) {
            _inv[i] = 1LL * _inv[MODULO % i].x * (MODULO - MODULO / i) % MODULO;
        }
        _fact[0] = 1;
        for (unsigned i = 1; i <= TABLE_SIZE; ++i) {
            _fact[i] = _fact[i - 1] * int(i);
        }
        _inv_fact[TABLE_SIZE] = _fact[TABLE_SIZE].inv();
        for (int i = (int)TABLE_SIZE - 1; i >= 0; --i) {
            _inv_fact[i] = _inv_fact[i + 1] * (i + 1);
        }
    }
};

template <int M, unsigned F>
std::ostream& operator<<(std::ostream& os, const mod_int<M, F>& rhs)
{
    return os << rhs.x;
}

template <int M, unsigned F>
std::istream& operator>>(std::istream& is, mod_int<M, F>& rhs)
{
    long long s;
    is >> s;
    rhs = mod_int<M, F>(s);
    return is;
}

template <int M, unsigned F>
mod_int<M, F> mod_int<M, F>::_inv[TABLE_SIZE + 1];

template <int M, unsigned F>
mod_int<M, F> mod_int<M, F>::_fact[TABLE_SIZE + 1];

template <int M, unsigned F>
mod_int<M, F> mod_int<M, F>::_inv_fact[TABLE_SIZE + 1];

template <int M, unsigned F>
bool operator==(const mod_int<M, F>& lhs, const mod_int<M, F>& rhs)
{
    return lhs.x == rhs.x;
}

template <int M, unsigned F>
bool operator!=(const mod_int<M, F>& lhs, const mod_int<M, F>& rhs)
{
    return !(lhs == rhs);
}

const int MF = 100005;  //割り算しないなら
const int MOD = 1000000007;

using mint = mod_int<MOD, MF>;

mint binom(int n, int r)
{
    return (r < 0 || r > n || n < 0) ? 0 : mint(n).choose(r);
}

mint fact(int n)
{
    return mint(n).fact();
}

mint inv_fact(int n)
{
    return mint(n).inv_fact();
}
void solve(long long N, long long K, std::vector<long long> A) {
  sort(A.begin(), A.end());
  mint ans = 0;
  map<lli,lli> ma;
  map<lli,lli> mb;
  map<lli,lli> mc;

  rep(i, N)
    ma[A[i]]++;
  lli tmp = 0;
  for(auto s:ma){
    mb[s.first] = tmp;
    tmp += s.second;
    mc[s.first] = N - tmp;
  }
  for(auto s:ma){
    ans -= mint(s.first)*(binom(mc[s.first]+s.second,K) - binom(mc[s.first], K));
    ans += mint(s.first) *(binom(mb[s.first]+s.second,K) - binom(mb[s.first], K)); 
  }
  cout << ans << endl;
}

int main() {
  int a;
  long long N;
  scanf("%lld", &N);
  long long K;
  scanf("%lld", &K);
  std::vector<long long> A(N);
  for (int i = 0; i < N; i++) {
    scanf("%lld", &A[i]);
  }
  solve(N, K, std::move(A));
  return 0;
}


Measured memory_bytes: 19584