#include <iostream>
#include <algorithm>
#include <numeric>
#include <cmath>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <functional>
using namespace std;
#define int long long
#define All(v) (v).begin(),(v).end()
int dy[8] = {-1,0,1,0,-1,1,1,-1};
int dx[8] = {0,1,0,-1,1,1,-1,-1};
const int mod = 1000000007;
const int inf = mod*mod;
const int d5 = 100100;

template <int mod>
struct ModInt
{
    int x;

    ModInt() : x(0) {}
    ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

    ModInt &operator+=(const ModInt &p)
    {
        if ((x += p.x) >= mod)
            x -= mod;
        return *this;
    }
    ModInt &operator-=(const ModInt &p)
    {
        if ((x += mod - p.x) >= mod)
            x -= mod;
        return *this;
    }
    ModInt &operator*=(const ModInt &p)
    {
        x = (int)(1LL * x * p.x % mod);
        return *this;
    }
    ModInt &operator/=(const ModInt &p)
    {
        *this *= p.inverse();
        return *this;
    }

    ModInt operator-() const { return ModInt(-x); }
    ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
    ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
    ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
    ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

    bool operator==(const ModInt &p) const { return x == p.x; }
    bool operator!=(const ModInt &p) const { return x != p.x; }

    ModInt inverse() const
    {
        int a = x, b = mod, u = 1, v = 0, t;
        while (b > 0)
        {
            t = a / b;
            a -= t * b;
            swap(a, b);
            u -= t * v;
            swap(u, v);
        }
        return ModInt(u);
    }

    ModInt pow(int e)
    {
        long long a = 1, p = x;
        while (e > 0)
        {
            if (e % 2 == 0)
            {
                p = (p * p) % mod;
                e /= 2;
            }
            else
            {
                a = (a * p) % mod;
                e--;
            }
        }
        return ModInt(a);
    }

    friend ostream &operator<<(ostream &os, const ModInt<mod> &p)
    {
        return os << p.x;
    }
    friend istream &operator>>(istream &is, ModInt<mod> &a)
    {
        long long x;
        is >> x;
        a = ModInt<mod>(x);
        return (is);
    }
};

using modint = ModInt<mod>;
modint f[d5];

void makef()
{
    f[0] = 1;
    for (int i = 1; i < d5; i++)
        f[i] = f[i - 1] * i;
}

modint nCr(int n, int r)
{
    if (n < r || r < 0)
        return 0;
    return f[n] / (f[r] * f[n - r]);
}
signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    makef();
    int n,k;
    cin>>n>>k;
    vector<int> a(n);
    for(int i=0;i<n;i++)cin>>a[i];
    sort(All(a));
    modint msum=0,Msum=0,z,A;
    for(int i=0;i<n;i++){
        z=nCr(n-i-1,k-1);
        A=a[i];
        //cout<<i<<' '<<z<<' '<<A<<endl;
        msum+=A*z;
    }
    for(int i=0;i<n;i++){
        A=a[i];
        z=nCr(i,k-1);
        Msum+=A*z;
    }
    modint res=Msum-msum;
    cout<<res<<endl;
}

Measured memory_bytes: 1792