#include <bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
 
#define IOS ios :: sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define AfterDecimal(n) cout << fixed << setprecision(n);
#define pb push_back
#define pp(a, b) pair <a, b>
#define ll long long int
#define ld long double
#define all(a) a.begin(), a.end()
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
#define itr iterator
#define bits(x) __builtin_popcount(x)
#define fori(n)  for (ll i = 0; i < n; i++)
#define forj(n)  for (ll j = 0; j < n; j++)
#define fork(n)  for (ll k = 0; k < n; k++)
#define vll vector <ll>
#define mll map <ll, ll>
#define vpll vector <pll>
 
 
using namespace __gnu_pbds;
using namespace std;
 
template <class T> T printr(T p){ cout << p << '\n'; return 0; }
template <class T> T printe(T p){ cout << p << '\n'; exit(0); }
template <class T> bool uin(T &a, T b) { return a > b ? (a = b, true) : false; }
template <class T> bool uax(T &a, T b) { return a < b ? (a = b, true) : false; }
 
 
//template <typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
 
typedef tree<int, null_type, less<int>, rb_tree_tag,  
            tree_order_statistics_node_update>  
    ordered_set;  
 
#ifdef LOCAL
#define debug(...) __f(#__VA_ARGS__, __VA_ARGS__)
template<typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cout << "# "<< name << " = " << arg1 << '\n';
}
template<typename Arg1, typename... Args>
void __f(const char* names, Arg1 && arg1, Args &&... args){
        const char* comma = strchr(names + 1 ,',');
        cout << "# ";
        cout.write(names, comma - names)<< " = " << arg1 << " | ";
        __f(comma + 1, args...);
}
#else
#define debug(...) 36
#endif
 
ll power(ll x, ll y, ll p){ 
   ll res = 1;
   x = x % p;  
   while (y > 0) {
      if (y & 1) 
         res = (res * x) % p; 
      y = y >> 1;
      x = (x * x) % p; 
   }
   return res;
}
 
const ll INF = (ll)(8e18);
const ll MOD = (ll)(1e9 + 7);
const ll HMOD = (ll)4222234727;
const ll hashprime = (ll)31;
 
const int maxn = 200010;

ll a[maxn], ncr[maxn];

int32_t main(){
    IOS;
    #ifndef ONLINE_JUDGE
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    #endif
    ll n,k; cin >> n >> k;
    for(ll i = 1; i <= n; i++) cin >> a[i];
    if(k == 1){
        cout << 0 << '\n';
        return 0;
    }
    sort(a + 1, a + n + 1);
    ll ans = 0;
    ll vu = 1, vd = 1;
    ll N = n - 2LL;
    ll K = k - 2LL;
    for(ll i = 0; i < K; i++){
        vu = vu * (N - i);
        vu %= MOD;
        vd = vd * (i + 1);
        vd %= MOD;
    }
    vd = power(vd, MOD - 2, MOD);   
    for(ll i = N; i >= K; i--){
        ncr[i] = (vu * vd) % MOD;
        vu = (vu * power(i, MOD - 2, MOD)) % MOD;
        vu = (vu * (i - K)) % MOD;
    }
    ll val = 0;
    for(ll i = N; i >= K; i--){
         val += (ncr[i]);
         val %= MOD;
    }
   // cout << -454547 % 4 << '\n';
    for(ll i = N; i >= K; i--){
        ans = ans - (a[N - i + 1] * val) % MOD;
        ans += MOD;
        ans %= MOD;
        val -= ncr[i];
        val += MOD;
        val %= MOD;
    }
    val = 0;
    for(ll i = N; i >= K; i--){
         val += (ncr[i]);
         val %= MOD;
    }
    for(ll i = n; i >= k; i--){
        ans = ans + (val % MOD * a[i]) % MOD;
        ans += MOD;
        ans %= MOD;
        val -= ncr[i - 2];
        val += MOD;
        val %= MOD;
    }
    cout << ans << '\n';
    #ifdef LOCAL
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
    #endif
}


Measured memory_bytes: 1408