#include <bits/stdc++.h>
#define SZ(v) int((v).size())
#define ALL(vec) begin(vec), end(vec)
using namespace std;
typedef long long i64;
template<typename T> inline bool uax(T &x, T y) {return (y > x) ? x = y, true : false;}
template<typename T> inline bool uin(T &x, T y) {return (y < x) ? x = y, true : false;}
#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); \
  stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
string to_string(char c) { return "'" + string(1, c) + "'";}
string to_string(string s) { return '"' + s + '"';}
string to_string(const char* s) { return to_string((string) s);}
template<typename A> string to_string(A);
template<typename A, typename B> string to_string(pair<A, B> p){
  return "(" + to_string(p.first) + ": " + to_string(p.second) + ")";}
template<typename A> string to_string(A v) {bool f = false; string r = "{"; 
  for (auto x: v) {if (f) r += ", "; r += to_string(x); f = true;} return r += "}";}
template<typename A> string to_string(vector<vector<A>> v) {string r; 
  for (auto x: v) r += "\n" + to_string(x); return r;}
void err(istream_iterator<string>) { cerr << endl; }
template<typename T,typename... Args> void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " =: " << to_string(a) << "; "; err(++it, args...); }
template<typename T> void kek(T ans) {cout << ans << endl; exit(0);}
int const MOD = 1e9 + 7;
long long const INF = 1e18 + 42;
/*************************************/
inline int add(int a, int b, int mod = MOD) {
  a += b; return a >= mod ? a - mod : a;
}
inline int sub(int a, int b, int mod = MOD) {
  a -= b; return a < 0 ? a + mod : a;
}
inline int mul(int a, int b, int mod = MOD) {
  return int((long long) a * b % mod);
}
inline int mpow(int base, long long ex, int mod = MOD) {
  int res = 1;
  for (; ex > 0; ex >>= 1) {
    if (ex & 1) res = mul(res, base, mod);
    base = mul(base, base, mod);
  }
  return res;
}
inline int inv(int a, int mod = MOD) {
  return mpow(a, mod - 2, mod);
}
inline int mdiv(int a, int b, int mod = MOD) {
  return mul(a, mpow(b, mod - 2,  mod));
}
inline void adds(int &a, int b, int mod = MOD) {
  a += b; if (a >= mod) a -= mod;
}
inline void subs(int &a, int b, int mod = MOD) {
  a -= b; if (a < 0) a += mod;
}
inline void muls(int &a, int b, int mod = MOD) {
  a = int((long long) a * b % mod);
}
inline void mdivs(int &a, int b, int mod = MOD) {
  a = mdiv(a, b, mod);
}
vector<int> fac, ifac;
inline int ncr(int n, int r) {
  if (n < r || r < 0 || n < 0) return 0;
  return mul(fac[n], mul(ifac[n - r], ifac[r]));
}
void prepare_factorial(int nax = 1e6 + 10) {
  fac.resize(nax);
  ifac.resize(nax);
  fac[0] = 1;
  for (int i = 1; i < nax; ++i) fac[i] = mul(i, fac[i - 1]);
  ifac[nax - 1] = inv(fac[nax - 1]);
  for (int i = nax - 1; i > 0; --i) ifac[i - 1] = mul(i, ifac[i]);
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  
  prepare_factorial();
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  for (int &x : a) cin >> x;
  sort(ALL(a));
  int ans = 0;
  for (int i = k - 1; i < n; ++i) {
    if (a[i] < 0) a[i] += MOD;
    adds(ans, mul(a[i], ncr(i, k - 1)));
  }
  reverse(ALL(a));
  for (int i = k - 1; i < n; ++i) {
    if (a[i] < 0) a[i] += MOD;
    subs(ans, mul(a[i], ncr(i, k - 1)));
  }
  kek(ans);
}


Measured memory_bytes: 8448