#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
#define inf (int)(1e9+7)
#define INF (ll)(1e18)
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { if (a < b) { a = b; return 1; } return 0; }
template<typename T> T gcd(T a, T b) { if (b == 0) return a; return gcd(b, a % b); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template<typename T> T pow(T a, int b) { return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1; }




const int mod = 1000000007;

struct mint {
    ll x;
    mint (ll x = 0) : x(x % mod) {}
    mint operator-() const { return mint(-x);}
    mint& operator += (const mint a) { // コンストラクタの、modで割った余りがくる
        if ((x += a.x) >= mod) x -= mod;
        return *this;
    }
    mint& operator -= (const mint a) {
        if ((x += -a.x + mod) >= mod) x -= mod;
        return *this;
    }
    mint& operator *= (const mint a) {
        (x *= a.x) %= mod;
        return *this;
    }
    mint operator + (const mint a) const {
        mint res(*this);
        return res += a; // ?
    }
    mint operator - (const mint a) const {
        mint res(*this);
        return res -= a;
    }
    mint operator * (const mint a) const {
        mint res(*this);
        return res *= a;
    }
    mint pow(ll t) const { // mint(a).pow(t) tは余りとったりしたらあかん
        if (t == 0) return 1;
        mint a = pow(t >> 1); // 再帰
        a *= a;
        if (t & 1) a *= *this;
        return a;
    }
    
    // 逆元
    mint inv() const {
        return pow(mod - 2);
    }
    mint& operator /= (const mint a) { // invを利用
        return (*this) *= a.inv();
    }
    mint operator / (const mint a) const {
        mint res(*this);
        return res /= a;
    }
    friend ostream &operator<<(ostream &s, mint a) { return s << a.x; }
};

const int MAX = 510000;
const int MOD = 1000000007;

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 n, k;
vector<int> a;

void solve() {
    cin >> n >> k;
    
    a.resize(n);
    
    rep(i, n) cin >> a[i];
    sort(all(a));
    
    COMinit();
    
    mint ans = 0;
    rep(ri, 2) {
        rep(i, n) {
            mint now = COM(i, k - 1);
            if (ri == 1) now = -now;
            ans += now * a[i];
        }
        reverse(a.begin(), a.end());
    }
    
    cout << ans << endl;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    
    int T;
    // cin >> T;
    T = 1;
    
    while (T--) {
        solve();
    }
}









Measured memory_bytes: 12672