#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using Vi = vector<int>;
using Vl = vector<ll>;
using Pii = pair<int, int>;
using Pll = pair<ll, ll>;

constexpr int I_INF = (1 << 30);
constexpr ll L_INF = (1ll << 62);

//==================================

template <std::int_fast64_t Modulus>
class ModInt {
    using i64 = std::int_fast64_t;

    // friend
    template <std::int_fast64_t _MOD>
    friend std::istream& operator>>(std::istream& is, ModInt<_MOD>& rhs);
    template <std::int_fast64_t _MOD>
    friend std::ostream& operator<<(std::ostream& os, const ModInt<_MOD>& rhs);

private:
    i64 a;

public:
    /**
     * コンストラクタ
     */
    constexpr ModInt(const i64 x = 0) noexcept : a(x % Modulus) {
        if (a < 0) a += Modulus;
    }

    constexpr i64& value() noexcept { return a; }
    constexpr const i64& value() const noexcept { return a; }

    constexpr ModInt operator+(const ModInt& rhs) const noexcept {
        return ModInt(*this) += rhs;
    }
    constexpr ModInt operator-(const ModInt& rhs) const noexcept {
        return ModInt(*this) -= rhs;
    }
    constexpr ModInt operator*(const ModInt& rhs) const noexcept {
        return ModInt(*this) *= rhs;
    }
    constexpr ModInt operator/(const ModInt& rhs) const noexcept {
        return ModInt(*this) /= rhs;
    }

    constexpr ModInt& operator+=(const ModInt rhs) noexcept {
        a += rhs.a;
        if (a >= Modulus) {
            a -= Modulus;
        }
        return *this;
    }
    constexpr ModInt& operator-=(const ModInt rhs) noexcept {
        if (a < rhs.a) {
            a += Modulus;
        }
        a -= rhs.a;
        return *this;
    }
    constexpr ModInt& operator*=(const ModInt rhs) noexcept {
        a = a * rhs.a % Modulus;
        return *this;
    }
    constexpr ModInt& operator/=(ModInt rhs) noexcept {
        i64 exp = Modulus - 2;
        while (exp) {
            if (exp & 1) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp >>= 1;
        }
        return *this;
    }

    constexpr ModInt& operator++() noexcept {
        *this += 1;
        return *this;
    }
    constexpr ModInt operator++(int) noexcept {
        ModInt res = *this;
        *this += 1;
        return res;
    }
    constexpr ModInt& operator--() noexcept {
        *this -= 1;
        return *this;
    }
    constexpr ModInt operator--(int) noexcept {
        ModInt res = *this;
        *this -= 1;
        return res;
    }

    constexpr bool operator<(const ModInt& rhs) const noexcept {
        return a < rhs.a;
    }
    constexpr bool operator>(const ModInt& rhs) const noexcept {
        return a > rhs.a;
    }
    constexpr bool operator<=(const ModInt& rhs) const noexcept {
        return a <= rhs.a;
    }
    constexpr bool operator>=(const ModInt& rhs) const noexcept {
        return a >= rhs.a;
    }
    constexpr bool operator==(const ModInt& rhs) const noexcept {
        return a == rhs.a;
    }

    constexpr ModInt pow(i64 k) const noexcept {
        ModInt res = 1;
        ModInt tmp = *this;
        for (; k > 0; k >>= 1) {
            if (k & 1) res *= tmp;
            tmp *= tmp;
        }
        return res;
    }

    constexpr ModInt inverse() const noexcept {
        i64 q = a;
        i64 b = Modulus, u = 1, v = 0;
        while (b) {
            i64 t = q / b;
            q -= t * b;
            swap(q, b);
            u -= t * v;
            swap(u, v);
        }
        u %= Modulus;
        if (u < 0) u += Modulus;
        return u;
    }
};

// iostream用の関数[ModInt]
template <std::int_fast64_t Modulus>
std::istream& operator>>(std::istream& is, ModInt<Modulus>& rhs) {
    is >> rhs.a;
    return is;
}
template <std::int_fast64_t Modulus>
std::ostream& operator<<(std::ostream& os, const ModInt<Modulus>& rhs) {
    os << rhs.a;
    return os;
}

//=======================================================================================================
//=======================================================================================================
//=======================================================================================================
//=======================================================================================================
//=======================================================================================================

// 二項係数(mod)
// 要ModInt
template <std::int_fast64_t Modulus>
class ModComb {
    using i64 = int_fast64_t;
    using Vl = std::vector<i64>;

private:
    Vl fac, inv, finv;

public:
    ModComb(i64 n) : fac(n + 1), inv(n + 1), finv(n + 1) {
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[1] = 1;
        for (i64 i = 2; i < n + 1; ++i) {
            fac[i] = fac[i - 1] * i % Modulus;
            inv[i] = Modulus - inv[Modulus % i] * (Modulus / i) % Modulus;
            finv[i] = finv[i - 1] * inv[i] % Modulus;
        }
    }

    ModInt<Modulus> comb(i64 n, i64 k) {
        ModInt<Modulus> res;
        if (n < k)
            res = 0;
        else if (n < 0 || k < 0)
            res = 0;
        else
            res = fac[n] * (finv[n - k] * finv[k] % Modulus) % Modulus;
        return res;
    }

    ModInt<Modulus> get_fac(i64 n) { return ModInt<Modulus>(fac[n]); }
};

constexpr ll MOD = (ll)1e9 + 7;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N, K;
    cin >> N >> K;

    Vl a(N);
    for (int i = 0; i < N; ++i) {
        cin >> a[i];
    }

    sort(a.begin(), a.end());

    Vl bo, num;
    ll tmp = a[0];
    ll tmp_num = 1;
    for (int i = 1; i < N; ++i) {
        if (a[i] == tmp) {
            ++tmp_num;
        }
        else {
            bo.push_back(tmp);
            num.push_back(tmp_num);
            tmp = a[i];
            tmp_num = 1;
        }
    }

    bo.push_back(tmp);
    num.push_back(tmp_num);

    int len = (int)bo.size();

    Vl num_sum(len + 1);
    for (int i = 0; i < len; ++i) {
        num_sum[i + 1] = num_sum[i] + num[i];
    }

    ModComb<MOD> mc(N);

    vector<ModInt<MOD>> smax(len), smin(len);
    for (int i = 0; i < len; ++i) {
        ll b = num_sum[len] - num_sum[i + 1];
        ll a = num_sum[i] - num_sum[0];
        if (N - b < K) {
            smax[i] = 0;
        }
        else if (a < K) {
            smax[i] = mc.comb(N - b, K);
        }
        else {
            smax[i] = mc.comb(N - b, K) - mc.comb(a, K);
        }
        if (N - a < K) {
            smin[i] = 0;
        }
        else if (b < K) {
            smin[i] = mc.comb(N - a, K);
        }
        else {
            smin[i] = mc.comb(N - a, K) - mc.comb(b, K);
        }
    }

    ModInt<MOD> ans = 0;
    for (int i = 0; i < len; ++i) {
        ans += (smax[i] - smin[i]) * bo[i];
    }

    cout << ans << endl;

    return 0;
}

Measured memory_bytes: 7020