/***************************************************
 * Problem Name : E - Max-Min Sums.cpp
 * Problem Link : https://atcoder.jp/contests/abc151/tasks/abc151_e
 * OJ           : atCoder
 * Verdict      : Trying
 * Date         : 2020-01-12
 * Problem Type : ABC - E
 * Author Name  : Saikat Sharma
 * University   : CSE, MBSTU
 ***************************************************/
#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <climits>
#include <cstring>
#include <string>
#include <sstream>
#include <vector>
#include <queue>
#include <list>
#include <unordered_map>
#include <unordered_set>
#include <cstdlib>
#include <deque>
#include <stack>
#include <bitset>
#include <cassert>
#include <map>
#include <set>
#include <cassert>
#include <iomanip>
#include <random>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

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

#define __FastIO ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define __FileRead freopen ("input.txt", "r", stdin)
#define __FileWrite freopen ("output.txt", "w", stdout)
#define SET(a,v) memset(a,v,sizeof(a))
#define SZ(v) (int)v.size()
#define pii pair<int,int>
#define pil pair <int, ll>
#define pli pair <ll, int>
#define pll pair <ll, ll>
#define debug cout <<"######\n"
#define debug1(x) cout <<"### " << x << " ###\n"
#define debug2(x,y) cout <<"# " << x <<" : "<< y <<" #\n"
#define nl cout << "\n";
#define sp cout << " ";
#define sl(n) scanf("%lld", &n)
#define sf(n) scanf("%lf", &n)
#define si(n) scanf("%d", &n)
#define ss(n) scanf("%s", n)
#define pf(n) scanf("%d", n)
#define pfl(n) scanf("%lld", n)
#define all(v) v.begin(), v.end()
#define rall(v) v.begin(), v.end()
#define srt(v) sort(v.begin(), v.end())
#define r_srt(v) sort(v.rbegin(), v.rend())
#define rev(v) reverse(v.begin(), v.end())
#define Sqr(x) ((x)*(x))
#define Mod(x, m) ((((x) % (m)) + (m)) % (m))
#define max3(a, b, c) max(a, max(b, c))
#define min3(a, b, c) min(a, min(b, c))
#define pb push_back
#define mk make_pair
#define MAX 1000005
#define INF 1000000009
#define MOD 1000000007

template<class T>
using min_heap = priority_queue<T, std::vector<T>, std::greater<T>>;
template<typename T>
using ordered_set  = tree<T, null_type, less<T>, rb_tree_tag,
      tree_order_statistics_node_update>;

template <typename T> string toString ( T Number ) {
    stringstream ss;
    ss << Number;
    return ss.str();
}
template<typename T> int toInt (T str) {
    stringstream ss;
    ss << str;
    int num;
    ss >> num;
    return num;
}
ll lcm (ll a, ll b) {
    return a * b / __gcd (a, b);
}
/************************************ Code Start Here ******************************************************/
ll f[MAX];
ll invf[MAX];
ll modpow (ll a, ll b) {
    ll res = 1;

    while (b) {
        if (b & 1)
            res = (res * a) % MOD;

        a = (a * a) % MOD;
        b >>= 1;
    }

    return res;
}
ll ncr (ll n, ll r) {
	if(r>n) return 0LL;
    if (r == 0 || n == r) return 1LL;

    return ( ( (f[n] * invf[n - r]) % MOD) * invf[r]) % MOD;
}
void fibo() {
    f[0] = 1;

    for (int i = 1; i < MAX; i++) {
        f[i] = (f[i - 1] * i) % MOD;
        invf[i] = modpow (f[i], MOD - 2);
    }
}
int main () {
    __FastIO;
    //~ cout << setprecision (10);
    //~ cout << fixed;
    fibo();
    int n, k;
    cin >> n >> k;
    vector<ll>ar (n);

    for (int i = 0; i < n; i++) {
        cin >> ar[i];
    }

    sort (all (ar) );
    //~ ll sum_max = 0, sum_min = 0;
    ll sum = 0;
    for (int i = 0; i < n; i++) {
		
		sum = (sum + (ar[i] * (ncr(i,k-1) - ncr(n-i-1, k-1)))%MOD)%MOD;
		
        //~ if (i >= k - 1) sum_max = (sum_max + (ar[i] * ncr (i, k - 1) ) % MOD ) % MOD;

        //~ if (i <= n - k) sum_min = (sum_min + (ar[i] * ncr (n - i - 1,
                                                  //~ k - 1) ) % MOD ) % MOD;
    }

    //~ cout << sum_max << " " << sum_min << " -\n";
    //~ cout << (sum_max - sum_min) << "\n";
    cout << sum << "\n";
    return 0;
}


Measured memory_bytes: 16640