#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define VIN(v) for(auto&elem_: (v) )cin>>elem_
#define VOUT(v, sep) for(_loop_int idx=0; idx<(_loop_int)v.size(); idx++) {cout<<v[idx]; if(idx<(_loop_int)v.size()-1) cout<<sep;} cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG2(x,y) cout<<#x<<": "<<x<<" "<<#y<<": "<<y<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(debugidx,v.size())cout<<" "<<v[debugidx];cout<<endl
#define DEBUG_ARR(v,n) cout<<#v<<":";REP(debugidx,n)cout<<" "<<v[debugidx];cout<<endl

const ll MOD = 1000000007ll;
const int IINF = numeric_limits<int>::max()/2-1;
const ll LINF = numeric_limits<ll>::max()/2-1;

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

template< int mod >
struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if((x += p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int) (1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

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

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while(n > 0) {
      if(n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt< mod >(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt< MOD >;

template< typename T >
struct Combination {
  vector< T > _fact, _rfact, _inv;

  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for(int i = 1; i <= sz; i++) _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for(int i = sz - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
    for(int i = 1; i <= sz; i++) _inv[i] = _rfact[i] * _fact[i - 1];
  }

  inline T fact(int k) const { return _fact[k]; }

  inline T rfact(int k) const { return _rfact[k]; }

  inline T inv(int k) const { return _inv[k]; }

  T P(int n, int r) const {
    if(r < 0 || n < r) return 0;
    return fact(n) * rfact(n - r);
  }

  T C(int p, int q) const {
    if(q < 0 || p < q) return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }

  T H(int n, int r) const {
    if(n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

int main() {
    ios::sync_with_stdio(false); cin.tie(0);
    int n,k;
    cin>>n>>k;
    vi a(n);
    VIN(a);
    sort(ALL(a));

    if(k==1){
        cout<<"0"<<endl;
        return 0;
    }

    Combination<modint> cb(200000);

  	//Sum( max X ) - Sum( min X )を考える
    modint ans=0;
    modint mnsum=0;
    modint mxsum=0;
    REP(i,n-k+1){
        modint tmp;
        tmp = a[i];
        tmp *= cb.C(n-i-1, k-1);
        mnsum += tmp;
    }

    FOR(j,k-1,n){
        modint tmp;
        tmp = a[j];
        tmp *= cb.C(j, k-1);
        mxsum += tmp;
    }

    ans = mxsum;
    ans -= mnsum;
    cout << ans.x << endl;
    return 0;
}

Measured memory_bytes: 5992