#pragma GCC optimize ("O3")
#include "bits/stdc++.h"

using namespace std;
using ll = long long int;

#define debugos cout
#define debug(v) {printf("L%d %s > ",__LINE__,#v);debugos<<(v)<<endl;}
#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:(v)){debugos<<e<<" ";}debugos<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){debugos<<(m)[x]<<" ";}debugos<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){debugos<<(m)[y][x]<<" ";}debugos<<endl;}}
#define all(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(auto cnt=remove_reference<decltype(l)>::type();(cnt)<(l);++(cnt))
#define rrepeat(cnt,l) for(auto cnt=(l)-1;0<=(cnt);--(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define diterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);--(cnt))
const ll MD = 1000000007ll; const long double PI = 3.1415926535897932384626433832795L;
inline void assert_call(bool assertion, function<void()> f) { if (!assertion) { cerr << "assertion fault:" << endl; f(); abort(); } }
template<typename T1, typename T2> inline ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << '(' << p.first << ':' << p.second << ')'; return o; }
template<typename Vec> inline ostream& _ostream_vecprint(ostream& os, const Vec& a) {
  os << '['; for (const auto& e : a) os << ' ' << e << ' '; os << ']'; return os; }
template<typename T> inline ostream& operator<<(ostream& o, const vector<T>& v) { return _ostream_vecprint(o, v); }
template<typename T, size_t S> inline ostream& operator<<(ostream& o, const array<T, S>& v) { return _ostream_vecprint(o, v); }
template<typename T> inline T& chmax(T& to, const T& val) { return to = max(to, val); }
template<typename T> inline T& chmin(T& to, const T& val) { return to = min(to, val); }
void bye(string s, int code = 0) { cout << s << endl; exit(code); }
mt19937_64 randdev(8901016);
template<typename T, typename Random = decltype(randdev), typename enable_if<is_integral<T>::value>::type* = nullptr>
inline T rand(T l, T h, Random& rand = randdev) { return uniform_int_distribution<T>(l, h)(rand); }
template<typename T, typename Random = decltype(randdev), typename enable_if<is_floating_point<T>::value>::type* = nullptr>
inline T rand(T l, T h, Random& rand = randdev) { return uniform_real_distribution<T>(l, h)(rand); }

#if defined(_WIN32) || defined(_WIN64)
#define getc_x _getc_nolock
#define putc_x _putc_nolock
#elif defined(__GNUC__)
#define getc_x getc_unlocked
#define putc_x putc_unlocked
#else
#define getc_x getc
#define putc_x putc
#endif
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
class MaiScanner {
  FILE* fp_;
public:
  inline MaiScanner(FILE* fp):fp_(fp){}
  template<typename T> void input_integer(T& var) noexcept {
    var = 0; T sign = 1;
    int cc = getc_x(fp_);
    for (; cc < '0' || '9' < cc; cc = getc_x(fp_))
      if (cc == '-') sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = getc_x(fp_))
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
  }
  inline int c() noexcept { return getc_x(fp_); }
  inline MaiScanner& operator>>(int& var) noexcept { input_integer<int>(var); return *this; }
  inline MaiScanner& operator>>(long long& var) noexcept { input_integer<long long>(var); return *this; }
  inline MaiScanner& operator>>(string& var) {
    int cc = getc_x(fp_);
    for (; !isvisiblechar(cc); cc = getc_x(fp_));
    for (; isvisiblechar(cc); cc = getc_x(fp_))
      var.push_back(cc);
    return *this;
  }
  template<typename IT> inline void in(IT begin, IT end) { for (auto it = begin; it != end; ++it) *this >> *it; }
};
class MaiPrinter {
  FILE* fp_;
public:
  inline MaiPrinter(FILE* fp):fp_(fp){}
  template<typename T>
  void output_integer(T var) noexcept {
    if (var == 0) { putc_x('0', fp_); return; }
    if (var < 0)
      putc_x('-', fp_),
      var = -var;
    char stack[32]; int stack_p = 0;
    while (var)
      stack[stack_p++] = '0' + (var % 10),
      var /= 10;
    while (stack_p)
      putc_x(stack[--stack_p], fp_);
  }
  inline MaiPrinter& operator<<(char c) noexcept { putc_x(c, fp_); return *this; }
  inline MaiPrinter& operator<<(int var) noexcept { output_integer<int>(var); return *this; }
  inline MaiPrinter& operator<<(long long var) noexcept { output_integer<long long>(var); return *this; }
  inline MaiPrinter& operator<<(char* str_p) noexcept { while (*str_p) putc_x(*(str_p++), fp_); return *this; }
  inline MaiPrinter& operator<<(const string& str) {
    const char* p = str.c_str();
    const char* l = p + str.size();
    while (p < l) putc_x(*p++, fp_);
    return *this;
  }
  template<typename IT> void join(IT begin, IT end, char sep = ' ') { for (bool b = 0; begin != end; ++begin, b = 1) b ? *this << sep << *begin : *this << *begin; }
};
MaiScanner scanner(stdin);
MaiPrinter printer(stdout);

template <int MaxN, long long Mod>
class Combinatorics {
  using value_type = long long;
  value_type fact_[MaxN + 1];
  // value_type inv_[MaxN + 1]; // iranai

 public:
  constexpr value_type pow(value_type x, value_type p) const {
    value_type y = 1;
    x = x % Mod;
    for (; 0 < p; p >>= 1) {
      if (p & 1)
        y = y * x % Mod;
      x = x * x % Mod;
    }
    return y;
  }

  constexpr Combinatorics() : fact_() {
    fact_[0] = 1;
    fact_[1] = 1;
    for (value_type n = 2; n <= MaxN; ++n)
      fact_[n] = (fact_[n - 1] * n) % Mod;
    // inv_[Max] := calculate;
    // iterate => inv_[i] = inv_[i+1] * (i+1) % MD;
  }
  constexpr value_type fact(int n) const { return fact_[n]; }

  constexpr value_type inv(value_type n) const { return pow(n, MD - 2); }

  constexpr value_type nPr(value_type n, value_type r) const {
    if (r < 0 || n < r)
      return 0;
    return fact_[n] * inv(fact_[n - r]) % MD;
  }
  constexpr value_type nCr(value_type n, value_type r) const {
    if (n < r)
      return 0;
    return ((fact_[n] * inv(fact_[n - r]) % MD) * inv(fact_[r])) % MD;
  }
  constexpr value_type nHr(value_type n, value_type r) const {
    if (n == 0 && r == 0)
      return 1;
    if (n == 0)
      return 0;
    return nCr(n - 1 + r, n - 1);
  }
};

//

int N, K;
int A[100010];

//

Combinatorics<100100, MD> comb;

int main() {
  
  scanner >> N >> K;
  scanner.in(A, A+N);
  sort(A, A+N);
  
  ll total = 0;
  repeat(i, N) {
    ll a = A[i];
    // a が max となるのは、左側からすべて選ぶ場合
    if ((i+1) >= K)
      total += a*comb.nCr(i, K-1);
    // a が max となるのは、右側からすべて選ぶ場合
    if ((N-i)+1 >= K)
      total -= a*comb.nCr(N-i-1, K-1);
    total %= MD;
    total += MD;
    total %= MD;
  }
  
  cout << total << endl;
  
  return 0;
}


Measured memory_bytes: 640