#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <stack>
#include <queue>
#include <cmath>
#include <tuple>
#include <cstdio>
#include <bitset>
#include <sstream>
#include <iterator>
#include <numeric>
#include <map>
#include <cstring>
#include <set>
#include <functional>
#include <iomanip>

using namespace std;

//#define DEBUG_ //!!提出時にコメントアウト!!
#ifdef DEBUG_
	#define dump(x)  cerr << #x << " = " << (x) << endl;
#else
	#define dump(x)  ;
#endif

#define equals(a,b) (fabs((a)-(b)) < EPS)
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define SZ(x) ((int)(x).size())
#define pb push_back
#define eb emplace_back

//#define int long long

typedef long long LL;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

template <typename T>
std::string printVector(const std::vector<T> &data)
{
    std::stringstream ss;
    std::ostream_iterator<T> out_it(ss, ", ");
    ss << "[";
    std::copy(data.begin(), data.end() - 1, out_it);
    ss << data.back() << "]";
    return ss.str();
}

template <typename T>
void print_array(const T &ary, int size){
    REP(i,size){
        cout << ary[i] << " ";
    }
    cout << endl;
}

const int mod = 1e9+7;
const LL LINF = 1001002003004005006ll;
const int INF = 1001001001;
const double EPS = (1e-10);

struct mint {
    LL x; // typedef long long LL;
    mint(LL x=0):x(x >= 0 ? x%mod : x%mod+mod){}
    mint& operator+=(const mint a) {
      if ((x += a.x) >= mod) x -= mod;
      return *this;
    }
    mint& operator-=(const mint a) {
      if ((x += mod-a.x) >= mod) x -= mod;
      return *this;
    }
    mint& operator*=(const mint a) {
      (x *= a.x) %= mod;
      return *this;
    }
    mint operator+(const mint a) const {
      mint res(*this);
      return res+=a;
    }
    mint operator-(const mint a) const {
      mint res(*this);
      return res-=a;
    }
    mint operator*(const mint a) const {
      mint res(*this);
      return res*=a;
    }
    mint pow(LL t) const {
      if (!t) return 1;
      mint a = pow(t>>1);
      a *= a;
      if (t&1) a *= *this;
      return a;
    }

    // for prime mod
    mint inv() const {
      return pow(mod-2);
    }
    mint& operator/=(const mint a) {
      return (*this) *= a.inv();
    }
    mint operator/(const mint a) const {
      mint res(*this);
      return res/=a;
    }
  };

const int MOD = 1e9+7;
const int MAX = 5e5;

  long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

signed main(int argc, char const *argv[])
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << setprecision(12);

    int N,K; cin >> N >> K;
    VI A(N); REP(i,N) cin >> A[i];
    COMinit();
    sort(A.begin(),A.end());

    mint tmax = 0, tmin = 0;
    FOR(i,K-1,N){
        LL cnt = COM(i,K-1);
        tmax += (cnt * A[i]) % mod;
    }
    reverse(A.begin(),A.end());
    FOR(i,K-1,N){
        LL cnt = COM(i,K-1);
        tmin += (cnt * A[i]) % mod;
    }
    cout << (tmax - tmin).x << endl;


}


Measured memory_bytes: 12416