#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
const int mod = 1000000007;
//const int mod = 998244353;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint operator-() const { return mint(-x);}
  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 { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= 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 { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}
// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
struct combination {
  vector<mint> fact, ifact;
  combination(int n):fact(n+1),ifact(n+1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) fact[i] = fact[i-1]*i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) ifact[i-1] = ifact[i]*i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return fact[n]*ifact[k]*ifact[n-k];
  }
};

int main(){
    ios::sync_with_stdio(false);
    int n,k;
    cin >> n >> k;
    combination c(n);
    mint ans = 0;
    vector<int> a(n);
    map<int,int> num;
    rep(i,n) {
        int tmp;
        cin >> tmp;
        a[i] = tmp;
        if(num.count(tmp) == 0) num[tmp] = 1;
        else num[tmp]++;
    }
    sort(a.begin(),a.end());
    //max
    rep(i,n) {
        auto lower_num_it = upper_bound(a.begin(), a.end(), a[i]);
        int lower_num;
        if(lower_num_it == a.end()) lower_num = n-num[a[i]];
        else lower_num = lower_num_it - a.begin() - num[a[i]];
        for(int j = 1; j <= num[a[i]]; j++) {
            ans += c(num[a[i]], j) * c(lower_num, k-j) * a[i];
        }
        i+=num[a[i]]-1;
    }

    //min
    vector<int> a_minus(n);
    rep(i,n) a_minus[i] = -a[n-i-1];
    rep(i,n) {
        int a_m = -a_minus[i];
        auto lower_num_it = upper_bound(a_minus.begin(), a_minus.end(), -a_m);
        int lower_num;
        if(lower_num_it == a_minus.end()) lower_num = n-num[a_m];
        else lower_num = lower_num_it - a_minus.begin() - num[a_m];
        for(int j = 1; j <= num[a_m]; j++) {
            ans -= c(num[a_m], j) * c(lower_num, k-j) * a_m;
        }
        i+=num[a_m]-1;
    }
    cout << ans << endl;
}


Measured memory_bytes: 9664