#include <iostream>
#include <iomanip>
#include <cmath>
#include <functional>
#include <algorithm>
#include <numeric>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <deque>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <boost/range/irange.hpp>
#include <boost/container/static_vector.hpp>
#include <boost/utility/string_ref.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/math/common_factor_rt.hpp>

class input
{
    std::istream& cin;

public:
    input() : cin(std::cin)
    {
        std::cin.tie(nullptr);
        std::ios::sync_with_stdio(false);
    }

    template <class T>
    inline input& operator>>(T& var)
    {
        cin >> var;
        return *this;
    }
    template <class T>
    inline input& operator,(T& var)
    {
        cin >> var;
        return *this;
    }

    template <class T>
    inline T read()
    {
        T var;
        cin >> var;
        return var;
    }

    template <class InputIterator>
    inline void read(InputIterator head, std::size_t n)
    {
        for (std::size_t i = 0; i < n; ++i) {
            cin >> *head;
            ++head;
        }
    }

    template <class T>
    inline std::vector<T> read(std::size_t n)
    {
        std::vector<T> res;
        res.reserve(n);

        for (std::size_t i = 0; i < n; ++i) {
            T tmp;
            cin >> tmp;
            res.emplace_back(tmp);
        }
        return res;
    }

    template <class T, class F>
    inline void online(std::size_t n, F&& f)
    {
        T var;
        for (std::size_t i = 0; i < n; ++i) {
            cin >> var;
            f(var);
        }
    }
};
template <class T, std::size_t n>
std::istream& operator>>(std::istream& in, std::array<T, n>& var)
{
    for (std::size_t i = 0; i < n; ++i) {
        in >> var[i];
    }
    return in;
}

class output
{
    std::ostream& cout;

    template <class T, class... Args>
    void impl(T&& v, Args&&... args)
    {
        cout << std::forward<T>(v) << ' ';
        impl(std::forward<Args>(args)...);
    }
    template <class T>
    void impl(T&& v)
    {
        cout << std::forward<T>(v);
    }

public:
    output() : cout(std::cout)
    {
        cout << std::fixed << std::setprecision(15);
    }

    template <class... Args>
    void operator()(Args&&... args)
    {
        impl(std::forward<Args>(args)...);
        cout << '\n';
    }
    template <class... Args>
    void noreturn(Args&&... args)
    {
        impl(std::forward<Args>(args)...);
    }

    template <class T>
    void print(const T& container)
    {
        bool first = true;
        for (auto&& x : container) {
            if (!first) {
                cout << ' ';
            }
            cout << x;
            first = false;
        }
        cout << '\n';
    }
};
// ax+by=gcd(a,b)となるx,yを求める
std::uint64_t extGCD(std::int64_t a, std::int64_t b, std::int64_t& x, std::int64_t& y)
{
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    auto d = extGCD(b, a % b, y, x);
    y -= a / b * x;
    return d;
}

template <std::uint64_t m = 1000000000 + 7>
class ModInt
{
    std::uint64_t v;

public:
    static constexpr auto mod = m;

public:
    ModInt() : v(0){};
    ModInt(std::uint64_t x) : v(x % mod){};
    ModInt(const ModInt&) = default;
    ModInt(ModInt&&) = default;
    ModInt& operator=(const ModInt&) = default;
    ModInt& operator=(ModInt&&) = default;
    std::uint64_t value() const { return v; }

    ModInt& operator+=(const ModInt& rhs)
    {
        v += rhs.v;
        v %= mod;
        return *this;
    }
    ModInt& operator-=(const ModInt& rhs)
    {
        if (v >= rhs.v) {
            v -= rhs.v;
            return *this;
        }
        v = mod - (rhs.v - v);
        return *this;
    }
    ModInt& operator*=(const ModInt& rhs)
    {
        __uint128_t t = v;
        t *= rhs.v;
        v = t % mod;
        return *this;
    }
    friend ModInt operator+(const ModInt& lhs, const ModInt& rhs)
    {
        auto t = lhs;
        t += rhs;
        return t;
    }
    friend ModInt operator-(const ModInt& lhs, const ModInt& rhs)
    {
        auto t = lhs;
        t -= rhs;
        return t;
    }
    friend ModInt operator*(const ModInt& lhs, const ModInt& rhs)
    {
        auto t = lhs;
        t *= rhs;
        return t;
    }

    ModInt power(const uint64_t& p) const
    {
        if (p == 0) return 1;
        if (p == 1) return v;
        auto t = power(p / 2);
        t *= t;
        if (p % 2) t *= v;
        return t;
    }

    ModInt inv() const
    {
        std::int64_t x, y;
        extGCD(v, mod, x, y);
        return x % mod + mod;
    }

    // invers [0,n]
    static const std::vector<ModInt>& inv(std::size_t n)
    {
        static std::vector<ModInt<>> buf(2, 1);
        if (buf.size() > n) {
            return buf;
        }
        auto old = buf.size();
        buf.resize(n + 1, 0);
        for (std::size_t i = old; i <= n; ++i) {
            buf[i] = (mod - mod / i) * buf[mod % i];
        }
        return buf;
    }

    bool operator<(const ModInt& rhs) const
    {
        return v < rhs.v;
    }
};

template <std::uint64_t mod>
std::ostream& operator<<(std::ostream& os, const ModInt<mod>& v)
{
    return os << '[' << v.value() << ']';
}

ModInt<> binomial(ModInt<> n, ModInt<> r)
{
    // static std::map<std::pair<ModInt<>, ModInt<>>, ModInt<>> buf;
    // {
    //     auto it = buf.find({n, r});
    //     if (it != buf.end()) return it->second;
    // }
    // {
    //     auto it = buf
    // }
    ModInt<> res = 1;
    auto& inv = ModInt<>::inv(r.value());
    for (std::size_t i = 0; i < r.value(); ++i) {
        res *= n - i;
        res *= inv[i + 1];
    }
    return res;
}

void prog()
{
    input in;
    output out;

    std::uint64_t n, k;
    in >> n, k;
    if (k == 1) {
        out(0);
        return;
    }

    std::vector<ModInt<>> a(n + 1);
    {
        auto t = in.read<int64_t>(n);
        std::sort(std::begin(t), std::end(t));
        for (auto i : boost::irange<std::uint64_t>(0, n)) {
            // a[i + 1] = a[i] + (t[i] + 1000000000);
            a[i + 1] = a[i] + (t[i] < 0 ? ModInt<>::mod + t[i] : t[i]);
        }
    }
    // out.print(a);
    ModInt<> res = 0;
    ModInt<> cmb = 1;
    auto& inv = ModInt<>::inv(n + 1);
    for (auto i : boost::irange<uint64_t>(k, n + 1)) {
        // auto c = binomial(i - 2, k - 2);
        // out(n - i + 1, i - 1);
        res += cmb * ((a[n] - a[i - 1]) - a[n - i + 1]);
        cmb *= i - 1;
        cmb *= inv[i - k + 1];
    }
    out(res.value());
}

int main()
{
    prog();
    return 0;
}


Measured memory_bytes: 1792