#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, j, n) for (int i = j; i < n; i++)
#define all(x) (x).begin(),(x).end()
#define pii pair<int, int>
#define pll pair<ll, ll>
#define vc vector
#define a first
#define b second
#define pb push_back
#define INF (1000000000)
#define MOD (1000000007)
#define MAX (5000000)

/////////////////////////////////////////////////////////

class XY{
public:
  ll x, y;
  XY() {x = y = 0;}
  XY(ll u, ll v) {x = u; y = v;}
};

template<typename T1, typename T2>
void chmin(T1 &a, T2 b) {if(a > b) a = b;}
template<typename T1, typename T2>
void chmax(T1 &a, T2 b) {if(a < b) a = b;}

template<typename T1, typename T2>
ll mypow(T1 a, T2 n){
  if(n == 0) return 1;
  if(n == 1) return a;
  if(n % 2) return a * mypow(a, n - 1);
  ll tmp = mypow(a, n / 2);
  return tmp * tmp;
}

template<typename T>
int BS(vector<T> &V, int left, int right, T key){
  int mid = (left + right) / 2;
  if(V[mid] <= key) left = mid;
  else right = mid;
  if(right - mid == 1) return left;
  else return BS(V, left, right, key);
}

ll comb(ll n, ll r){
	ll res = 1;
  rep(i, 0, r){
    res *= n - i;
    res /= i + 1;
  }
  return res;
}

template<typename T>
T euclid(T a, T b){
  if(b == 0) return a;
  else return euclid(b, a % b);
}

/////////////////////////////////////////////////////////

//MOD of Combination

vector<ll> fact(MAX, 0), fact_inv(MAX, 0);

ll pow_mod(ll a, ll b){
	ll res = 1;
	while(b > 0){
		if(b & 1) res = res * a % MOD;
		a = a * a % MOD;
		b >>= 1;
	}
	return res;
}

void prepare_comb_mod(ll n){
	fact[0] = 1LL;
	for(ll i = 0; i < n; i++) fact[i+1] = fact[i] * (i + 1) % MOD;
	fact_inv[n] = pow_mod(fact[n], MOD - 2);
	for(ll i = n - 1; i >= 0; i--) fact_inv[i] = fact_inv[i + 1] * (i + 1) % MOD;
}

ll comb_mod(ll n, ll r){
	return (fact[n] * fact_inv[r]) % MOD * fact_inv[n - r] % MOD;
}

/////////////////////////////////////////////////////////

//Union-Find Tree

template<class T> struct UFT{
  vector<int> par;
  vector<int> rank;
  vector<T> diff_weight;

  UFT(int n = 1, T w = 0) {
    init(n, w);
  }

  void init(int n = 1, T w = 0) {
    par.resize(n);
    rank.resize(n);
    diff_weight.resize(n);
    rep(i, 0, n){
      par[i] = i;
      rank[i] = 0;
      diff_weight[i] = w;
    }
  }

  int root(int x){
    if(par[x] == x){
      return x;
    }else{
      int r = root(par[x]);
      diff_weight[x] += diff_weight[par[x]];
      return par[x] = r;
    }
  }

  bool unite(int x, int y, int w = 0){
    w += weight(x); w -= weight(y);
    x = root(x);
    y = root(y);
    if(x == y) return false;
    if(rank[x] < rank[y]){
      par[x] = y;
      diff_weight[x] = -w;
    }else{
      par[y] = x;
      diff_weight[y] = w;
      if(rank[x] == rank[y]) rank[x]++;
    }
    return true;
  }

  bool same(int x, int y){
    return root(x) == root(y);
  }

  T weight(int x) {
    root(x);
    return diff_weight[x];
  }

  T diff(int x, int y) {
    return weight(y) - weight(x);
  }
};

/////////////////////////////////////////////////////////

ll digit(ll N){
    return log10(N)+1;
}

/////////////////////////////////////////////////////////

/////////////////////////////////////////////////////////

void Main () {
  ll N, K; cin >> N >> K;
  vc<ll> A(N);
  rep(i, 0, N) cin >> A[i];
  sort(all(A), greater<ll>());
  prepare_comb_mod(N);
  ll res = 0;
  rep(i, 0, N - K + 1){
    res += A[i] * comb_mod(N - i - 1, K - 1);
    res %= MOD;
  }
  rep(i, 0, N - K + 1){
    res -= A[N - i - 1] * comb_mod(N - i - 1, K - 1);
    res %= MOD;
    if(res < 0){
      res += MOD;
    }
  }
  cout << res << endl;
}

/////////////////////////////////////////////////////////

int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << std::fixed << std::setprecision(15);

  Main();
}


Measured memory_bytes: 79232