// May this submission get accepted
#pragma GCC optimize ("O3")

#include <bits/stdc++.h>

// 汎用マクロ
#define ALL_OF(x) (x).begin(), (x).end()
#define REP(i,n) for (long long i=0, i##_len=(n); i<i##_len; i++)
#define RANGE(i,is,ie) for (long long i=(is), i##_end=(ie); i<=i##_end; i++)
#define DSRNG(i,is,ie) for (long long i=(is), i##_end=(ie); i>=i##_end; i--)
#define STEP(i, is, ie, step) for (long long i=(is), i##_end=(ie), i##_step = (step); i<=i##_end; i+=i##_step)
#define UNIQUE(v) do { sort((v).begin(), (v).end()); (v).erase(unique((v).begin(), (v).end()), (v).end()); } while (false)
template<class T> bool chmax(T &a, const T &b) { if (a < b) {a = b; return true;} return false; }
template<class T> bool chmin(T &a, const T &b) { if (a > b) {a = b; return true;} return false; }
#define INF 0x7FFFFFFF
#define LINF 0x7FFFFFFFFFFFFFFFLL
#define Yes(q) ((q) ? "Yes" : "No")
#define YES(q) ((q) ? "YES" : "NO")
#define Possible(q) ((q) ? "Possible" : "Impossible")
#define POSSIBLE(q) ((q) ? "POSSIBLE" : "IMPOSSIBLE")
#define DUMP(q) cerr << "[DEBUG] " #q ": " << (q) << " at " __FILE__ ":" << __LINE__ << endl
#define DUMPALL(q) do { cerr << "[DEBUG] " #q ": ["; REP(DUMPALL_ITR, (q).size()) { cerr << (q)[DUMPALL_ITR] << (DUMPALL_ITR == DUMPALL_ITR_len-1 ? "" : ", "); } cerr << "] at " __FILE__ ":" << __LINE__ << endl; } while (false)
template<class T> T gcd(T a, T b) { if (a < b) std::swap(a, b); while (b) std::swap(a %= b, b); return a; }
template<class T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

// gcc拡張マクロ
#define popcount __builtin_popcount
#define popcountll __builtin_popcountll

// エイリアス
#define DANCE_ long
#define ROBOT_ unsigned
#define HUMAN_ signed
using  ll = DANCE_ HUMAN_ DANCE_;
using ull = DANCE_ ROBOT_ DANCE_;
using  ld = long double;
using namespace std;

// 標準入出力
struct inp {
    size_t sz;
    inp(size_t _sz = 1) : sz(_sz) {}
    template <typename T> operator T () const { T a; cin >> a; return a; }
    template <typename T> operator vector<T> () const { vector<T> a(sz); for (size_t i = 0; i < sz; i++) cin >> a[i]; return a; }
    template <typename T, typename U> operator pair<T, U> () const { T f; U s; cin >> f >> s; return pair<T, U>(f, s); }
};
inp inp1; // input one
template <typename T> void say(T &x, char end = '\n') { cout << x << end; }
template <typename T> void say(vector<T> &x, char sep = ' ', char end = '\n') { for (size_t i = 0, sz = x.size(); i < sz; i++) { cout << x[i] << (i == sz-1 ? end : sep); } }

// モジュール

// い　つ　も　の
constexpr long long pdiv = 1000000007LL;
// constexpr long long pdiv = 998244353LL;
struct modint {
    using ll = long long;
    ll x = 0;
    modint(ll _x = 0): x(_x) { compile(); }
    void compile() { x = (x % pdiv + pdiv) % pdiv; }
    modint pow(long long n) const {
        long long result = 1;
        long long base = x;
        long long mask = 1;
        while (mask <= n) {
            if (n & mask) (result *= base) %= pdiv;
            mask <<= 1;
            (base *= base) %= pdiv;
        }
        return modint(result);
    }
    modint inv() const { return this->pow(pdiv-2); }
    modint operator+ (long long a) const { return modint(x + a); }
    modint operator- (long long a) const { return modint(pdiv + x - a); }
    modint operator* (long long a) const { return modint(x * a); }
    modint operator/ (long long a) const { return modint(x * modint(a).inv().x); }
    modint operator+ (modint a) const { return modint(x + a.x); }
    modint operator- (modint a) const { return modint(pdiv + x - a.x); }
    modint operator* (modint a) const { return modint(x * a.x); }
    modint operator/ (modint a) const { return modint(x * a.pow(pdiv-2).x); }
    template <typename T> modint& operator+= (T a) { return *this = *this + a; }
    template <typename T> modint& operator-= (T a) { return *this = *this - a; }
    template <typename T> modint& operator*= (T a) { return *this = *this * a; }
    template <typename T> modint& operator/= (T a) { return *this = *this / a; }
    modint operator- () const { return modint(pdiv - x); }
    template <typename T>
    modint& operator= (T a) { x = a; compile(); return *this; }
};
modint operator+ (const long long a, const modint b) { return modint(a) + b; }
modint operator- (const long long a, const modint b) { return modint(a) - b; }
modint operator* (const long long a, const modint b) { return modint(a) * b; }
modint operator/ (const long long a, const modint b) { return modint(a) / b; }
modint operator"" _p(unsigned long long _x) { return modint(_x); }
ostream& operator<< (ostream& ost, const modint a) { ost << a.x; return ost; }
istream& operator>> (istream& ist, modint &a) { ist >> a.x; return ist; }
modint pow(modint a, long long n) { return a.pow(n); }
vector<modint> fac_cache = {1}, invfac_cache = {1};
void prepare_fac(long long nmax) {
    long long m0 = fac_cache.size() - 1;
    if (m0 < nmax) {
           fac_cache.resize(nmax+1);
        invfac_cache.resize(nmax+1);
    }
    for (long long n = m0+1; n <= nmax; n++) {
        fac_cache[n] = fac_cache[n-1] * n;
        invfac_cache[n] = fac_cache[n].inv();
    }
}
modint    fac(long long n) { prepare_fac(n); return    fac_cache[n]; }
modint invfac(long long n) { prepare_fac(n); return invfac_cache[n]; }
bool isinpascal(long long n, long long r) { return n >= 0 && r >= 0 && n >= r; }
modint nCr(long long n, long long r) { return !isinpascal(n, r) ? 0_p : fac(n) * invfac(r) * invfac(n-r); }
modint nPr(long long n, long long r) { return !isinpascal(n, r) ? 0_p : fac(n) * invfac(n-r); }
modint nHr(long long n, long long r) { return nCr(n+r-1, r); }

// 処理内容
int main() {
    
    ios::sync_with_stdio(false); // stdioを使うときはコメントアウトすること
    cin.tie(nullptr);            // インタラクティブ問題ではコメントアウトすること

    ll n = inp1, k = inp1;
    vector<ll> a = inp(n);
    sort(ALL_OF(a));

    modint ans = 0;
    REP(i, n) {
        ans += a[i] * nCr(i, k-1);
        ans -= a[i] * nCr(n-1-i, k-1);
    }
    say(ans);

}

Measured memory_bytes: 3064