#include <bits/stdc++.h>
using i64 = int_fast64_t;
#define repeat(i, a, b) for(int i = (a); (i) < (b); ++(i))
#define rep(i, n) repeat(i, 0, n)

namespace edamat {  // modint: mod 計算を int を扱うように扱える構造体
template <int MOD>
struct Fp {
    long long val;
    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if(val < 0) val += MOD;
    }
    constexpr int getmod() { return MOD; }
    constexpr Fp operator-() const noexcept {
        return val ? MOD - val : 0;
    }
    constexpr Fp operator+(const Fp& r) const noexcept { return Fp(*this) += r; }
    friend constexpr Fp<MOD> operator+(const i64 lhs, const Fp& r) noexcept { return r + lhs; }
    constexpr Fp operator-(const Fp& r) const noexcept { return Fp(*this) -= r; }
    friend constexpr Fp<MOD> operator-(const i64 lhs, const Fp& r) noexcept { return -r + lhs; }
    constexpr Fp operator*(const Fp& r) const noexcept { return Fp(*this) *= r; }
    friend constexpr Fp<MOD> operator*(const i64 lhs, const Fp& r) noexcept { return r * lhs; }
    constexpr Fp operator/(const Fp& r) const noexcept { return Fp(*this) /= r; }
    constexpr Fp& operator+=(const Fp& r) noexcept {
        val += r.val;
        if(val >= MOD) val -= MOD;
        return *this;
    }
    constexpr Fp& operator-=(const Fp& r) noexcept {
        val -= r.val;
        if(val < 0) val += MOD;
        return *this;
    }
    constexpr Fp& operator*=(const Fp& r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }
    constexpr Fp& operator/=(const Fp& r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while(b) {
            long long t = a / b;
            a -= t * b;
            std::swap(a, b);
            u -= t * v;
            std::swap(u, v);
        }
        val = val * u % MOD;
        if(val < 0) val += MOD;
        return *this;
    }
    constexpr bool operator==(const Fp& r) const noexcept {
        return this->val == r.val;
    }
    constexpr bool operator!=(const Fp& r) const noexcept {
        return this->val != r.val;
    }
    friend constexpr std::ostream& operator<<(std::ostream& os, const Fp<MOD>& x) noexcept {
        return os << x.val;
    }
    friend constexpr Fp<MOD> modpow(const Fp<MOD>& a, i64 n) noexcept {
        if(n == 0) return 1;
        auto t = modpow(a, n / 2);
        t = t * t;
        if(n & 1) t = t * a;
        return t;
    }
};

// 二項係数ライブラリ
template <class T>
struct BiCoef {
    std::vector<T> fact_, inv_, finv_;
    constexpr BiCoef() {}
    constexpr BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) {
        init(n);
    }
    constexpr void init(int n) noexcept {
        fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1);
        int MOD = (1e9 + 7);
        for(int i = 2; i < n; i++) {
            fact_[i] = fact_[i - 1] * i;
            inv_[i] = -inv_[MOD % i] * (MOD / i);
            finv_[i] = finv_[i - 1] * inv_[i];
        }
    }
    constexpr T com(int n, int k) const noexcept {
        if(n < k || n < 0 || k < 0) return 0;
        return fact_[n] * finv_[k] * finv_[n - k];
    }
    constexpr T fact(int n) const noexcept {
        if(n < 0) return 0;
        return fact_[n];
    }
    constexpr T inv(int n) const noexcept {
        if(n < 0) return 0;
        return inv_[n];
    }
    constexpr T finv(int n) const noexcept {
        if(n < 0) return 0;
        return finv_[n];
    }
};
}  // namespace edamat

const int MOD = 1000000007;
using modint = edamat::Fp<MOD>;
edamat::BiCoef<modint> bc;

void solve() {
    int n, k;
    std::cin >> n >> k;
    std::vector<int> a(n);
    rep(i, n) std::cin >> a[i];
    std::sort(begin(a), end(a));
    bc.init(n + 10);
    modint ans = 0;
    rep(i, n) {  // maxを足していく
        ans += a[i] * bc.com(i, k - 1);
    }
    rep(i, n) {  // minを引いていく
        ans -= a[i] * bc.com(n - i - 1, k - 1);
    }
    std::cout << ans << std::endl;
}

int main() {
    // std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    solve();
    return 0;
}

Measured memory_bytes: 2944