#include <bits/stdc++.h>
 
using namespace std;

#define M 1000000007
#define pb emplace_back
#define f first
#define s second
#define rep(i, st, ed) for(ll i=st; i<ed; ++i)
#define repn(i, st, ed) for(ll i=st; i<=ed; ++i)
#define repb(i, ed, st) for(ll i=ed; i>=st; --i)
#define all(v) v.begin(),v.end()
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

typedef long long ll;
typedef unsigned long long int llt;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef vector<ll> vec;
typedef vector<pll> vecp;
typedef map<ll, ll> mpll;

const long double PI  = 3.141592653589793238463;
const int N = 1e5 + 10;
const ll INF = 1LL << 60;

template<class T> void chmax(T &a,T b) { if (a<b) a=b;}
template<class T> void chmin(T &a,T b) { if (a>b) a=b;}

template <typename A, typename B>
string to_string(pair<A, B> p);
 
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
 
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
 
string to_string(const string& s) {
  return '"' + s + '"';
}
 
string to_string(const char* s) {
  return to_string((string) s);
}
 
string to_string(bool b) {
  return (b ? "true" : "false");
}
 
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
 
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
 
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
 
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
 
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}
 
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
 
void debug_out() { cerr << endl; }
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << to_string(H) << endl;
  debug_out(T...);
}
#define LOCAL 3
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:" << endl, debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

ll ppow(ll x, ll y, ll mod)
{
  ll res = 1;
  while(y > 0){
    if(y&1)
      res = (res * x)%mod;
    y = y >> 1;
    x = (x * x)%mod;
  }
  return res;
}

ll mul(ll x, ll y)
{
  return (((x%M)*(y%M))%M);
}
ll factdp[N];
ll fact(ll x)
{
  if(x == 0) return 1;
  if(x == 1) return 1;
  if(factdp[x] != 0) return factdp[x];
  return factdp[x] = (x*fact(x-1))%M;
}
// Combination
ll modcomb(ll n, ll r)
{
	if(n <= 0) return 0;
	if(r < 0) return 0;
  if(n < r) return 0;
  ll res = fact(n);
  (res *= ppow(fact(r), M-2, M)) %= M;
  (res *= ppow(fact(n-r), M-2, M)) %= M;
  return res;
}
void solve()
{
	ll n, k;
	cin>>n>>k;
	vector<ll> v(n);
	rep(i, 0, n) cin>>v[i];
	sort(all(v));
	ll ans = 0;
	rep(i, 0, n)
	{
		ans += (v[i] * modcomb(i, k-1)) % M;
	}
	rep(i, 0, n)
	{
		ans -= (v[i] * modcomb(n-i-1, k-1)) % M;
		while(ans < 0) ans += M;
	}
	ans = ans % M;
	cout<<ans<<endl;
}


int main()
{
	ll Tests = 1;
    //cin>>Tests;
    while(Tests--)
    {
    	solve();
    }
    return 0;
}

Measured memory_bytes: 2560