/**
 *  “Experience is the name everyone gives to their mistakes.” – Oscar Wilde
 *
 *  author  :   prodipdatta7
 *  created :   Thursday 16-January, 2020  10:28:57 AM
**/

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

// #pragma GCC optimize("Ofast")
// #pragma GCC target("avx,avx2,fma")
// #pragma GCC optimize("unroll-loops")

using namespace std ;
using namespace __gnu_pbds ;

#define     ll          long long
#define     ld          long double
#define     ull         unsigned long long
#define     pii         pair<int,int>
#define     pll         pair<ll,ll>
#define     vi          vector<int>
#define     vll         vector<ll>
#define     vvi         vector<vector<int>>
#define     debug(x)    cerr << #x << " = " << x << '\n' ;
#define     rep(i,b,e)  for(__typeof(e) i = (b) ; i != (e + 1) - 2 * ((b) > (e))  ; i += 1 - 2 * ((b) > (e)))
#define     all(x)      x.begin() , x.end()
#define     rall(x)     x.rbegin() , x.rend()
#define     sz(x)       (int)x.size()
#define     ff          first
#define     ss          second
#define     pb          push_back
#define     eb          emplace_back
#define     mem(a)      memset(a , 0 ,sizeof a)
#define     memn(a)     memset(a , -1 ,sizeof a)
#define     fread       freopen("input.txt","r",stdin)
#define     fwrite      freopen("output.txt","w",stdout)
#define     TN          typename

typedef tree <int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update > ordered_set;
typedef tree <int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update > ordered_multiset;

/* 
Note :  There is a problem with erase() function in ordered_multiset (for less_equal<int> tag). 
        lower_bound() works as upper_bound() and vice-versa. 
        Be careful to use.
        i) find_by_order(k) : kth smallest element counting from 0 .
        ii) order_of_key(k) : number of elements strictly smaller than k.
*/

/*###############-> Input Section <-###############*/

template <TN T> inline void Int(T &a) {
    bool minus = false; a = 0; char ch = getchar();
    while (true) { if (ch == '-' or (ch >= '0' && ch <= '9')) break; ch = getchar(); }
    if (ch == '-') minus = true; else a = ch - '0';
    while (true) { ch = getchar(); if (ch < '0' || ch > '9') break; a = a * 10 + (ch - '0'); }
    if (minus)a *= -1 ;
}
template < TN T, TN T1 > inline void Int(T &a, T1 &b) {Int(a), Int(b) ;}
template < TN T, TN T1, TN T2 > inline void Int(T &a, T1 &b, T2 &c) {Int(a, b), Int(c) ;}
template < TN T, TN T1, TN T2, TN T3 > inline void Int(T &a, T1 &b, T2 &c, T3 &d) {Int(a, b), Int(c, d) ;}
template < TN T, TN T1, TN T2, TN T3, TN T4> inline void Int(T &a, T1 &b, T2 &c, T3 &d, T4 &e) {Int(a, b), Int(c, d, e) ;}

/*###############-> Debugger <-###############*/

#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
void err(istream_iterator<string> it) {cout << endl ;}
template<TN T, TN... Args>
void err(istream_iterator<string> it, T a, Args... args) {
    cerr << *it << " = " << a << ' ' ;
    err(++it, args...);
}

/*###############-> Constraints <-###############*/

const int N          = (int)2e5 + 5 ;
const int maxN       = (int)1e6 + 6 ;
const ll  Mod        = (ll)1e9 + 7 ;
const int inf        = (int)2e9 ;
const ll  Inf        = (ll)1e18 ;
const int mod        = (int)1e9 + 7 ;

inline int add(int a, int b, int mod) {a += b ; return a >= mod ? a - mod : a ;}
inline int sub(int a, int b, int mod) {
    a -= b ; 
    while(a < 0)a += mod ;
    return a ;
}
inline int mul(int a, int b, int mod) {return (ll)a * b % mod ;}

/*...... ! Code starts from here ! ......*/

int F[N], a[N] ;

void Factorial(int n){
    F[0] = 1 ;
    for(int i = 1 ; i <= n ; ++i)
        F[i] = mul(F[i - 1], i, mod) ;
}

int modpow(int b, int p){
    int res = 1 ;
    while(p){
        if(p & 1)res = mul(res, b, mod) ;
        b = mul(b, b, mod) ;
        p >>= 1 ;
    }
    return res ;
}

int call(int n, int k){
    return mul(F[n], modpow(mul(F[k], F[n - k], mod), mod - 2), mod) ;
}

int main() {
    // ios_base::sync_with_stdio(false) ;
    // cin.tie(NULL) ; cout.tie(NULL) ;
    int test = 1 , tc = 0 ;
    //Int(test) ;
    while (test--) {
        int n, k ; Int(n, k) ;
        Factorial(n) ;
        for(int i = 1 ;  i <= n ; ++i){
            Int(a[i]) ;
        }
        sort(a + 1, a + n + 1) ;
        int res = 0 ;
        for(int i = 1 ; i <= n - k + 1 ; ++i){
            int nck = call(n - i, k - 1) ;
            int cur = sub(a[n - i + 1], a[i], mod) ;
            //error(nck, cur, n-i+1, i)
            res = add(res, mul(nck, cur, mod), mod) ;
        }
        printf("%d\n",res);
    }
    return 0 ;
}

Measured memory_bytes: 1024