#pragma region header
#include <bits/stdc++.h>
#define int long long
#define all(a) begin(a), end(a)
#define rall(a) rbegin(a), rend(a)
#define mp make_pair
#define mt make_tuple
#define rep1(i, n) for (decltype(+n) i = 0; i < (n); i++)
#define rrep1(i, n) for (auto i = n - 1; i > static_cast<decltype(i)>(-1); i--)
#define rep2(i, a, b) for (auto i = (a); i < (b); i++)
#define rrep2(i, a, b) for (auto i = b - 1; i >= a; i--)
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define rep(...) GET_MACRO(__VA_ARGS__, rep2, rep1) (__VA_ARGS__)
#define rrep(...) GET_MACRO(__VA_ARGS__, rrep2, rrep1) (__VA_ARGS__)
#define each(i, a) for (auto &&i : (a))
using namespace std;

using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vs = vector<string>;
using vvs = vector<vs>;
using vd = vector<ld>;
using vvd = vector<vd>;
using vb = vector<bool>;
using vvb = vector<vb>;
using pii = pair<int, int>;
using vp = vector<pii>;
using vvp = vector<vp>;
using mii = map<int, int>;
using vm = vector<mii>;
using vvm = vector<vm>;
template <class T>
using less_queue = priority_queue<T>;
template <class T>
using greater_queue = priority_queue<T, vector<T>, greater<T>>;

constexpr int INF = 1e18;
const int MOD = 1000000007;
int mod(int a) { return (a % MOD + MOD) % MOD; }
int m_add(int a, int b) { return (a + b) % MOD; }
int m_add(int a, int b, int c) { return (a + b + c) % MOD; }
int m_sub(int a, int b) { return (a + MOD - b) % MOD; }
int m_mul(int a, int b) { return a * b % MOD; }
int m_mul(int a, int b, int c) { return a * b % MOD * c % MOD; }
int m_bipow(int x, int y) {
  if (y == 0) return 1;
  else if (y == 1) return x % MOD;
  else if (y % 2 == 0) {
    int z = m_bipow(x, (int)(y / 2));
    return m_mul(z, z);
  } else {
    int z = m_bipow(x, (int)(y / 2));
    return m_mul(z, z, x);
  }
}
int m_inv(int x) { return m_bipow(x, MOD - 2); }
int m_div(int a, int b) { return m_mul(a, m_inv(b)); }

template <class T>
void SORT(T &a) { stable_sort(all(a)); }
template <class T>
void RSORT(T &a) { stable_sort(rall(a)); }
template <class T>
void rev(T &a) { reverse(rall(a)); }
template <class T>
void uniq(T &a) { a.erase(unique(all(a)), end(a)); }
template <class T>
auto min_of(T a) { return *min_element(all(a)); }
template <class T>
auto max_of(T a) { return *max_element(all(a)); }
template <class T>
int sum_of(T a) { return accumulate(all(a), 0ll); }
template <class T, class U>
auto sum_of(T a, U init) { return accumulate(all(a), init); }
template <class T, class U>
int count_of(T a, U i) { return count(all(a), i); }
template <class T, class U>
int lower_index(T a, U i) { return lower_bound(all(a), i) - begin(a); } // use member func for set
template <class T, class U>
int upper_index(T a, U i) { return upper_bound(all(a), i) - begin(a); }
template <class T, class U>
bool binary(T a, U i) { return binary_search(all(a), i); }
template <class T, class U>
bool has(T a, U i) { return find(all(a), i) != end(a); }
template <class T>
int sz(T a) { return a.size(); };

template <class T>
void COUT(T x) { cout << x << endl; }
template <class T, class U>
void COUT(T x, U y) { cout << x << ' ' << y << endl; }
template <class T, class U, class V>
void COUT(T x, U y, V z) { cout << x << ' ' << y << ' ' << z << endl; }
template <class T>
void CSP(T x) { cout << x << ' '; }
template <class T>
void CVEC(T v) {
  int c = v.size() - 1;
  for (size_t i = 0; i < c; i++) cout << v[i] << ' ';
  if (c > -1) cout << v[c];
  cout << endl;
}

template <class T>
bool amin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
bool amax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

int ceil_div(int x, int y) { return (x - 1) / y + 1; }

#pragma endregion header

// MOD = 1000000007;
struct combination {
  vi fact, ifact;
  combination(int n): fact(n + 1), ifact(n + 1) {
    assert(n < MOD);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) fact[i] = m_mul(fact[i - 1], i);
    ifact[n] = m_inv(fact[n]);
    for (int i = n; i >= 1; --i) ifact[i-1] = m_mul(ifact[i], i);
  }
  int operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return m_mul(fact[n], ifact[k], ifact[n - k]);
  }
} comb(100000);

void solve(int n, int k, vi a) {
  SORT(a);
  int ans = 0;
  rep(i, k - 1, n) ans = m_add(ans, m_mul(a[i], comb(i, k - 1)));
  rep(i, n - k + 1) ans = m_sub(ans, m_mul(a[i], comb(n - i - 1, k - 1)));
  COUT(ans);
}


#pragma region main
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout << fixed << setprecision(15);

  int n;
  cin >> n;
  int k;
  cin >> k;
  vi a(n);
  for(int i = 0 ; i < n ; i++){
    cin >> a[i];
  }
  solve(n, k, move(a));
}
#pragma endregion main


Measured memory_bytes: 6112