#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for(int i=(int)(s-1); i>=(int)(e); i--)
#define pb push_back
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1e18;
const ll MOD = 1e9 + 7;
const double EPS = 1e-8;

template<typename T> T chmax(T& a, const T& b){return a = (a > b ? a : b);}
template<typename T> T chmin(T& a, const T& b){return a = (a < b ? a : b);}

#define DEBUG_MODE
#ifdef DEBUG_MODE
#define dump(x) cout << #x << " : " << x << " "
#define dumpL(x) cout << #x << " : " << x << '\n'
#define LINE cout << "line : " << __LINE__ << " "
#define LINEL cout << "line : " << __LINE__ << '\n'
#define dumpV(v) cout << #v << " : ["; for(auto& t : v) cout << t << ", "; cout<<"]" << " "
#define dumpVL(v) cout << #v << " : ["; for(auto& t : v) cout << t << ", "; cout<<"]" << endl
#define STOP assert(false)
#else
#define dump(x) 
#define dumpL(x) 
#define LINE 
#define LINEL 
#define dumpV(v) 
#define dumpVL(v) 
#define STOP assert(false)
#endif
#define mp make_pair
 
namespace std {
template<class S, class T>
ostream &operator <<(ostream& out, const pair<S, T>& a) {
    out << '(' << a.fi << ", " << a.se << ')';
    return out;
}
}

template <typename T, typename U> T pow_fast(T x, U n) {
    T ret = (T)1;
    while(n) {
        if(n & (U)1) ret *= x;
        x *= x;
        n >>= 1;
    }
    return ret;
}

template <typename ModType, ModType MOD> struct ModInt {
    ModType val;
    ModInt() : val(0) {}
    ModInt(ModType x) : val(x % MOD) {
        if(val < 0) val += MOD;
    };
    operator ModType() const { return val; }
    ModInt &operator+=(const ModInt &m) {
        val += m.val;
        if(val >= MOD) val -= MOD;
        return *this;
    }
    ModInt &operator-=(const ModInt &m) {
        val -= m.val;
        if(val < 0) val += MOD;
        return *this;
    }
    ModInt &operator*=(const ModInt &m) {
        (val *= m.val) %= MOD;
        return *this;
    }
    ModInt &operator/=(const ModInt &m) {
        *this *= m.inverse();
        return *this;
    }
    ModInt operator+(const ModInt &m) const { return ModInt(*this) += m; }
    ModInt operator-(const ModInt &m) const { return ModInt(*this) -= m; }
    ModInt operator*(const ModInt &m) const { return ModInt(*this) *= m; }
    ModInt operator/(const ModInt &m) const { return ModInt(*this) /= m; }
    ModInt inverse() const { return pow_fast(*this, MOD - 2); }
    ModInt pow(ModType n) const {return pow_fast(*this, n); }

    friend std::ostream &operator<<(std::ostream &os, const ModInt &rhs) {
        os << rhs.val;
        return os;
    }
    friend std::istream &operator>>(std::istream &is, ModInt &rhs) {
        ModType value;
        is >> value;
        rhs = ModInt(value);
        return is;
    }
};

template <typename ModType, ModType MOD> struct Comb {
    using mint = ModInt<ModType, MOD>;
    vector<mint> fact;
    vector<mint> ifact;
    const int N;
    Comb(int n) : N(n + 1), fact(n + 1), ifact(n + 1) { makeFact(); }
    void makeFact() {
        fact[0] = ifact[0] = 1LL;
        for(int i = 1; i < N; i++) {
            fact[i] = fact[i - 1] * (mint)i;
            ifact[i] = fact[i].inverse();
        }
    }
    mint operator()(ModType n, ModType r) {
        if(n < 0 || r < 0 || r > n) return 0;
        if(r > n / 2) r = n - r;
        return fact[n] * ifact[n - r] * ifact[r];
    }
};

int main(){
    using mint = ModInt<ll, MOD>;
    Comb<ll, MOD> comb(3e5);
    int n, k;
    cin >> n >> k;
    vector<ll> v(n);
    rep(i, n) cin >> v[i];
    sort(all(v));
    vector<mint> a(n);
    rep(i, n) a[i] = mint(v[i]);
    mint ans = 0;
    {
        int x = k-1;
        for(int i = n - k; i >= 0; --i) {
            ans -= comb(x, k-1) * a[i];
            ++x;
            // dump(a[i]), dump(comb(x, k-1)), dumpL("");
        }
    }
    {
        int x = k - 1;
        for(int i = k-1; i < n; ++i) {
            ans += comb(x, k-1) * a[i];
            ++x;
        }
    }
    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 6528