#include <algorithm>
#include <iostream>
#include <vector>

#define ll long long

using namespace std;

const int MAX = 510000;
const int MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  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;
  }
}

long long COM(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;
}

int main() {
  ll N, K;
  cin >> N >> K;
  vector<ll> A(N);
  for (int i = 0; i < N; i++) {
    cin >> A[i];
  }
  sort(A.begin(), A.end());
  COMinit();

  ll ans = 0;
  for (int i = 0; i < N; i++) {
    ans += (A[i] * (COM(i, K - 1) - COM(N - i - 1, K - 1))) % MOD;
  }
  cout << ans % MOD << endl;
}
