#include <algorithm>
#include <cstdint>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include <cfloat>

using namespace std;

using lint=int64_t;
using uint=uint32_t;
using ulint=uint64_t;

template<class T>
using vector2d=vector<vector<T>>;

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

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

template <class T>
void OutVec(const vector<T>& vec){
  for(int i = 0; i < vec.size() - 1; ++i){
    cout<<vec[i]<<" ";
  }
  cout<<vec.back()<<endl;
}

template <class T>
void OutVec2d(const vector2d<T>& vec){
  for(auto v:vec){
    OutVec(v);
  }
}

class ModCombination{
  std::vector<int64_t> fac, finv, inv;
  int64_t max,mod;
public:
  ModCombination(int64_t max_,int64_t mod_):max(max_),mod(mod_),fac(max_+1),finv(max_+1),inv(max_+1){
    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;
    }
  }
  int64_t GetCombination(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;
  }

};

int64_t EuclidMod(int64_t v, int64_t m){
  if(0 <= v && v < m){
    return v;
  }else if(-m <= v && v < 0){
    return v + m;
  }else{
    int64_t result = v % m;
    if(result < 0){
      result += m;
    }
    return result;
  }
}

int64_t ModInv(int64_t a, int64_t m){
  int64_t b = m, u = 1, v = 0;
  while(b){
    int64_t t = a / b;
    a -= t * b;
    std::swap(a, b);
    u -= t * v;
    std::swap(u, v);
  }
  EuclidMod(u, m);
  return u;
}

// return base^exponent (MOD. mod).
int64_t RepeatedPowMod(int64_t base, int64_t exponent, int64_t mod) {
  if (exponent == 0)
    return 1;
  else if (exponent % 2 == 0) {
    int64_t root = RepeatedPowMod(base, exponent / 2, mod);
    return (root * root) % mod;
  } else {
    return (base * RepeatedPowMod(base, exponent - 1, mod)) % mod;
  }
}

template<int64_t mod>
class ModInt{
  int64_t value_;

public:
  explicit ModInt(const int64_t x = 0): value_(x % mod){
  };

  ModInt& operator=(const ModInt& another){
    value_=another.value_;
    return *this;
  }

  ModInt& operator^=(const ModInt& another){
    value_=RepeatedPowMod(value_,another.value_,mod);
    return *this;
  }

  ModInt& operator+=(const ModInt& another){
    value_ += another.value_;
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator-=(const ModInt& another){
    value_ -= another.value_;
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator*=(const ModInt& another){
    value_ *= another.value_;
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator/=(const ModInt& another){
    value_ = value_ * ModInv(another.value_, mod);
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator++(){
    value_ += 1;
    value_=EuclidMod(value_,mod);
    return *this;
  }

  ModInt operator++(int){
    ModInt tmp = *this;
    this->value_+=1;
    this->value_=EuclidMod(this->value_,mod);
    return tmp;
  }

  ModInt& operator--(){
    value_ -= 1;
    value_=EuclidMod(value_,mod);
    return *this;
  }

  ModInt operator--(int){
    ModInt tmp=*this;
    this->value_-=1;
    this->value_=EuclidMod(this->value_,mod);
    return tmp;
  }



  int64_t GetValue() const{
    return value_;
  }

};

template<int64_t mod>
ModInt<mod>
operator+(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) += right;
}

template<int64_t mod>
ModInt<mod>
operator-(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) -= right;
}

template<int64_t mod>
ModInt<mod>
operator/(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) /= right;
}

template<int64_t mod>
ModInt<mod>
operator*(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) *= right;
}

template<int64_t mod>
ModInt<mod>
operator^(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) ^= right;
}

template<int64_t mod>
std::ostream& operator<<(std::ostream& stream, const ModInt<mod>& mod_int){
  stream << mod_int.GetValue();
  return stream;
}

int main(){
  cout << std::fixed << std::setprecision(16);
  cin.tie(nullptr);
  std::ios::sync_with_stdio(false);

  lint n,k;
  cin>>n>>k;
  vector<lint> a(n);
  for(int i=0;i<n;i++){
    cin>>a[i];
  }

  std::sort(a.begin(), a.end());
  const int64_t MOD=1000000007;
  ModInt<MOD> result;
  ModCombination comb(n+1,MOD);

  for(int r=0;r<n;r++){
    result+=ModInt<MOD>(comb.GetCombination(r,k-1))*ModInt<MOD>(a[r]);
  }
  std::sort(a.begin(), a.end(), std::greater<>());
  for(int r=0;r<n;r++){
    result-=ModInt<MOD>(comb.GetCombination(r,k-1))*ModInt<MOD>(a[r]);
  }
  cout<<result<<endl;





  return 0;
}




Measured memory_bytes: 3456