#include<bits/stdc++.h>
#include<iostream>
#include<cstdio>
#include<vector>
#include<string>
#include<algorithm>
#include<map>
using namespace std;
#define rep(i,x) for(ll i = 0; i < (ll)(x); i++)
#define pb push_back
#define eb emplace_back
#define debug(x) cerr << #x << ": " << (x) << "\n";
#define all(x) (x).begin(), (x).end()
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;
typedef pair<ll,ll> Pll;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef vector<vector<vector<ll>>> vvvl;
const ll INF = numeric_limits<ll>::max()/4;
const ll MOD = 1e9+7;
const int n_max = 1e5+10;

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

    public:
    i64 a;

    constexpr modint(const i64 x = 0) noexcept {
        this -> a = x % Modulus;
        if(a < 0){
            a += Modulus;
        }
    }
    constexpr i64 &value() const 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 /= 2;
        }
        return *this;
    }
    
    // 自前実装
    constexpr bool operator==(const modint rhs) noexcept {
        return a == rhs.a;
    }
    constexpr bool operator!=(const modint rhs) noexcept {
        return a != rhs.a;
    }
    constexpr bool operator>(const modint rhs) noexcept {
        return a > rhs.a;
    }
    constexpr bool operator>=(const modint rhs) noexcept {
        return a >= rhs.a;
    }
    constexpr bool operator<(const modint rhs) noexcept {
        return a < rhs.a;
    }
    constexpr bool operator<=(const modint rhs) noexcept {
        return a <= rhs.a;
    }
    constexpr modint& operator++() noexcept {
        return (*this) += modint(1);
    }
    constexpr modint operator++(int) {
        modint tmp(*this);
        operator++();
        return tmp;
    }
    constexpr modint& operator--() noexcept {
        return (*this) -= modint(1);
    }
    constexpr modint operator--(int) {
        modint tmp(*this);
        operator--();
        return tmp;
    }
};

using mint = modint<MOD>;
// 標準入出力対応
std::ostream &operator<<(std::ostream &out, const modint<MOD> &m) {
    out << m.a;
    return out;
}
std::istream &operator>>(std::istream &in, modint<MOD> &m) {
    ll a;
    in >> a;
    m = mint(a);
    return in;
}

const int MAX = 1e5 + 10;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}


int main(){
    ll n,k; cin >> n >> k;
    vector<ll> a(n);
    rep(i,n) cin >> a[i];
    sort(all(a));
    COMinit();
    vector<mint> min_(n), max_(n);
    mint ans = 0;
    rep(i,n){
        max_[i] = COM(i, k-1);
        min_[i] = COM(n-1-i, k-1);
    }
    rep(i,n){
        ans += max_[i] * mint(a[i]);
        ans -= min_[i] * mint(a[i]);
    }
    cout << ans << endl;
}

Measured memory_bytes: 4992