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

const ll MOD = 1e9 + 7;

template<ll MOD> struct ModInt {
    ll value;
    ModInt() : value(0) {}
    ModInt(const ll& v) : value(v % MOD) {
        if ( value < 0 ) value += MOD;
    }
    ModInt<MOD>& operator=(const ModInt& o) {
        value = o.value;
        return *this;
    }
    ModInt<MOD>& operator+=(const ModInt& o) {
        value += o.value;
        if ( value >= MOD ) value -= MOD;
        return *this;
    }
    ModInt<MOD>& operator-=(const ModInt& o) {
        value += MOD - o.value;
        if (value >= MOD ) value -= MOD;
        return *this;
    }
    ModInt<MOD>& operator*=(const ModInt& o) {
        value *= o.value;
        value %= MOD;
        return *this;
    }
    ModInt<MOD>& operator/=(const ModInt& o) {
        value *= inverse(o.value);
        value %= MOD;
        return *this;
    }
    ModInt<MOD> operator-() {
        return ModInt<MOD>(-value);
    }
    ModInt<MOD> operator+(const ModInt& o) {
        ModInt<MOD> r(*this);
        r += o;
        return r;
    }
    ModInt<MOD> operator-(const ModInt& o) {
        ModInt<MOD> r(*this);
        r -= o;
        return r;
    }
    ModInt<MOD> operator*(const ModInt& o) {
        ModInt<MOD> r(*this);
        r *= o;
        return r;
    }
    ModInt<MOD> operator/(const ModInt& o) {
        ModInt<MOD> r(*this);
        r /= o;
        return r;
    }
    ModInt<MOD> pow(ll n) {
        ModInt<MOD> t = *this, r = 1;
        while ( n > 0 ) {
            if ( n & 1 ) r *= t;
            t *= t;
            n >>= 1;
        }
        return r;
    }
private:
    ll inverse(ll a) {
        ModInt<MOD> t(*this);
        return t.pow(MOD - 2);
    }
};

template<ll MOD> std::ostream& operator<< (std::ostream& os, const ModInt<MOD>& o) {
    os << o.value;
    return os;
}
template<ll MOD> std::istream& operator>> (std::istream& is, ModInt<MOD>& o) {
    is >> o.value;
    return is;
}

//---------------------------

template<typename T> struct Combination {
    ll n_max;
    vector<T> fac, finv, inv;
    Combination(ll n_max) {
        fac.resize(n_max+1);
        finv.resize(n_max+1, 0);
        inv.resize(n_max+1, 0);
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[0] = 0, inv[1] = 1;
        for (int i = 2; i <= n_max; i++){
            fac[i] = fac[i - 1] * i;
            inv[i] = - inv[MOD%i] * (MOD / i);
            finv[i] = finv[i - 1] * inv[i];
        }
    }

    // 二項係数
    T C(ll n, ll k) {
        if (n < k) return 0;
        if (n < 0 || k < 0) return 0;
        return fac[n] * finv[k] * finv[n - k];
    }
    T P(ll n, ll k) {
        if (n < k) return 0;
        if (n < 0 || k < 0) return 0;
        return fac[n] * finv[n - k];
    }
    T H(ll n, ll k) {
        return C(n+k-1, k);
    }

};

//---------------------------
using Int = ModInt<MOD>;

Int solve() {
    ll N, K;
    cin >> N >> K;
    vector<ll> A(N);
    for ( int i = 0; i < N; i++ ) {
        cin >> A[i];
    }
    sort(A.begin(), A.end());

    Combination<Int> cb(2*N);

    Int ans(0);
    for ( int i = 0; i < N; i++ ) {
        if ( N-1-i >= 0 ) {
            Int x(A[N-1-i]), y(A[i]);
            x *= cb.C(N-i-1, K-1);
            y *= cb.C(N-i-1, K-1);
            ans += x;
            ans -= y;
        }
    }
    return ans;
}

int main() {
    auto ans = solve();
    cout << ans << "\n";
    return 0;
}

Measured memory_bytes: 5760