//2019年12月8日現在

#include<stdio.h>
#include<cmath>
#include<math.h>
#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<complex>
#include<queue>
#include<set>
#include<utility>
#include<iomanip>
#include<limits>
#include<thread>
#include<map>
#include<stack>
#include<numeric>
#include<cassert>
#include<random>
#include<chrono>
#include<unordered_set>
#include<unordered_map>
#include<fstream>
#include<list>
#include<functional>
#include<bitset>
#include<tuple>
#include <cstdio>

using namespace std;

template <class... A>
void prints() { cout << "\n"; }
template <class... A>
void prints_rest() { cout << "\n"; }
template <class T, class... A>
void prints_rest(const T &first, const A &... rest) { cout << " " << first; prints_rest(rest...); }
template <class T, class... A>
void prints(const T &first, const A &... rest) { cout << first; prints_rest(rest...); }

namespace {
    // printElms(&a[0], a[n]);　みたいな感じで使う。
    void printElems(const int* begin, const int* end)
    {
        for (const int* p = begin; p != end; ++p) {
            std::cout << *p << ' ';
        }
        std::cout << std::endl;
    }
}



#define rep(i,n) for(int i=0; i<n; i++)
#define rep1(i,n) for(int i=1; i<n+1; i++)
#define repr(i,n) for(int i=n-1; i>=0; i--)
#define repr1(i,n) for(int i=n; i>0; i--)
#define souatari rep1(i,n-1)rep(j,i)
#define souatari1 for(int i=2; i<n+1; i++)rep1(j,i-1)
#define mp make_pair
#define F first
#define S second
#define elif else if

typedef long long ll;
typedef vector<int>  vi;
typedef vector<vi>  vvi;
typedef vector<ll> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pii> vpii;
typedef array<int,3> arr3;
typedef long double D;


const ll llinf =  numeric_limits<ll>::max();
const ll _llinf = numeric_limits<ll>::lowest();
const int inf = numeric_limits<int>::max();
const int _inf = numeric_limits<int>::lowest();
const double dinf = numeric_limits<double>::infinity();
const ll MOD = 1000000007;
const ll BIG = 10000000000000000;

template <unsigned P>
struct ModInt
{
        using M = ModInt;
        unsigned v;
        ModInt() : v(0) {}
        ModInt(auto x) : v(x >= 0 ? x % P : (P - -x % P) % P) {}
        constexpr ModInt(unsigned v, int) : v(v) {}
        static constexpr unsigned p() { return P; }
        M operator+() const { return *this; }
        M operator-() const { return {v ? P - v : 0, 0}; }
        explicit operator bool() const noexcept { return v; }
        bool operator!() const noexcept { return !(bool)*this; }
        M operator*(M r) const { return M(*this) *= r; }
        M operator/(M r) const { return M(*this) /= r; }
        M operator+(M r) const { return M(*this) += r; }
        M operator-(M r) const { return M(*this) -= r; }
        bool operator==(M r) const { return v == r.v; }
        bool operator!=(M r) const { return !(*this == r); }
        M &operator*=(M r)
        {
                v = (uint64_t)v * r.v % P;
                return *this;
        }
        M &operator/=(M r) { return *this *= r.inv(); }
        M &operator+=(M r)
        {
                if ((v += r.v) >= P)
                        v -= P;
                return *this;
        }
        M &operator-=(M r)
        {
                if ((v += P - r.v) >= P)
                        v -= P;
                return *this;
        }
        M inv() const
        {
                int a = v, b = P, x = 1, u = 0;
                while (b)
                {
                        int q = a / b;
                        swap(a -= q * b, b);
                        swap(x -= q * u, u);
                }
                assert(a == 1);
                return x;
        }
        M pow(auto n) const
        {
                if (n < 0)
                        return pow(-n).inv();
                M res = 1;
                for (M a = *this; n; a *= a, n >>= 1)
                        if (n & 1)
                                res *= a;
                return res;
        }
        friend M operator*(auto l, M r) { return M(l) *= r; }
        friend M operator/(auto l, M r) { return M(l) /= r; }
        friend M operator+(auto l, M r) { return M(l) += r; }
        friend M operator-(auto l, M r) { return M(l) -= r; }
        friend ostream &operator<<(ostream &os, M r) { return os << r.v; }
        friend istream &operator>>(istream &is, M &r)
        {
                ll x;
                is >> x;
                r = x;
                return is;
        }
        friend bool operator==(auto l, M r) { return M(l) == r; }
        friend bool operator!=(auto l, M r) { return !(l == r); }
};

const int MAX = 100005;
long long fac[MAX], finv[MAX], inv[MAX];

//必ずCOMinit()をmain関数に書くこと
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;
        }
}

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 a[100005];
int main(){
    COMinit();
    int n,k;
    cin >> n >> k;
    rep(i,n) cin >> a[i];
    sort(a,a+n);

    //printElems(&a[0],&a[n]);

    ModInt<MOD> ans = 0;

    for (int i = 1; n-i>=k-1 ; i++){
        ModInt<MOD> tmp;
        tmp -=a[i-1];
        tmp +=a[n-i];
        tmp*=COM(n-i,k-1);
        ans+=tmp;
    }

    prints(ans);



    //prints(ans);

    return 0;
}

Measured memory_bytes: 2944