//https://atcoder.jp/contests/abc151/tasks/abc151_e

#include <iostream>
#include <vector>
#include <set>
#include <queue>
#include <map>
#include <cassert>
#include <algorithm>
#include <functional>
#include <iomanip>
#include <limits>
#include <stack>
#include <math.h>
#include <cstdint>
#define REP(i,n) for(ll i=0, i##_len=(n); i<i##_len; ++i)
#define RREP(i,n) for(ll i=(n)-1; i>=0; --i)
#define SREP(i,s,n) for(ll i=(s), i##_len=(n); i<i##_len; ++i)
#define ALL(v) (v).begin(),(v).end()

using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;

template <typename T>
T gcd(T a, T b) {
    if(a < b) gcd(b, a);
    if (b == 1) return 1;
    T r;
    while ((r=a%b)) {
        a = b;
        b = r;
    }
    return b;
}

bool comp(pair<ll, ll> a, pair<ll, ll> b) {
    return a.second < b.second;
}

template<typename T>
struct UnionFind {
    vector<T> par;
    vector<T> rank;
    
    UnionFind(T n = 1) {
        init(n);
    }
    
    void init(T n = 1) {
        par.resize(n); rank.resize(n);
        for (T i = 0; i < n; ++i) {
            par[i] = i;
            rank[i] = 0;
        }
    }
    
    int root(T x) {
        if (par[x] == x) {
            return x;
        }
        else {
            int r = root(par[x]);
            return par[x] = r;
        }
    }
    
    bool issame(T x, T y) {
        return root(x) == root(y);
    }
    
    bool merge(T x, T y) {
        x = root(x); y = root(y);
        if (x == y) return false;
        if (rank[x] < rank[y]) swap(x, y);
        if (rank[x] == rank[y]) ++rank[x];
        par[y] = x;
        return true;
    }
};


#define INF (ll(1e18))

class SegmentTree {
public:
    int N, n;
    vector<ll> value;
    SegmentTree(int n) {
        this->n = n;
        this->N = 1;
        while (N < n) N *= 2;
        this->value = vector<ll>(2 * N - 1, INF);
    }
    
    void update(int i, ll x) {
        i += N - 1;
        value[i] = x;
        while (i > 0) {
            i = (i - 1) / 2;
            value[i] = min(value[i * 2 + 1], value[i * 2 + 2]);
        }
    }
    
    ll query(int l, int r) {
        return _query(l, r, 0, 0, N);
    }
    
    ll _query(int a, int b, int k, int l, int r) {
        if (r <= a || b <= l) return INF;
        if (a <= l && r <= b)
            return value[k];
        else {
            ll c1 = _query(a, b, 2 * k + 1, l, (l + r) / 2);
            ll c2 = _query(a, b, 2 * k + 2, (l + r) / 2, r);
            return min(c1, c2);
        }
    }
};

int msb(ll x) {
    int n = 0;
    while (x > 0) {
        x/=2;
        n++;
    }
    return n;
}


template <ll Modulus> class modint {
public:
    ll a;

    constexpr modint(const ll x = 0) noexcept : a(x % Modulus) {}
    constexpr ll &value() noexcept { return a; }
    constexpr operator int() const { return a; }
    constexpr operator long long() const { return a; }
    constexpr const ll &value() const noexcept { return a; }
    constexpr modint operator+(const modint rhs) const noexcept {
        return modint(*this) += rhs;
    }
    constexpr modint operator+(const ll rhs) const noexcept {
        return modint(*this) += rhs;
    }
    constexpr modint operator-(const modint rhs) const noexcept {
        return modint(*this) -= rhs;
    }
    constexpr modint operator-(const ll rhs) const noexcept {
        return modint(*this) -= rhs;
    }
    constexpr modint operator*(const modint rhs) const noexcept {
        return modint(*this) *= rhs;
    }
    constexpr modint operator*(const ll rhs) const noexcept {
        return modint(*this) *= rhs;
    }
    constexpr modint operator/(const modint rhs) const noexcept {
        return modint(*this) /= rhs;
    }
    constexpr modint operator/(const ll 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 {
        ll exp = Modulus - 2;
        while (exp) {
            if (exp % 2) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp /= 2;
        }
        return *this;
    }
};

#define MOD (1000000007)

int main(int argc, const char * argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    ll N, K;
    cin >> N >> K;
    vector<ll> A(N);
    REP(i, N) {
        cin >> A[i];
    }
    sort(ALL(A));
    vector<ll> S(N);
    S[0] = A[0];
    SREP(i, 1, N) {
        S[i] = S[i-1]+A[i];
    }
    
    vector<modint<MOD>> F(N+1);
    F[0] = 1;
    SREP(i, 1, N) {
        F[i] = F[i-1]*i;
    }
    
    // K = 1

    
    
    
    if (K == 1) {
        cout << 0 << endl;
    } else {
        modint<MOD> mi(0);
        SREP(x, K, N+1) {
            modint<MOD> y = F[x-2]/F[x-K]/F[K-2];
            y *= (S[N-1]-S[x-2])-S[N-x];
            
            mi += y;
        }
        cout << mi.value() << endl;
    }
    
    
}



Measured memory_bytes: 2560