#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
 
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
 
#define start_routine() int begtime = clock();
#define end_routine() int endtime = clock(); cerr << endl << "Time elapsed: " << (endtime - begtime)*1000/CLOCKS_PER_SEC << " ms"; return 0
#define speed() cin.tie(0), cout.tie(0), ios_base::sync_with_stdio(false)
// #define exit(a, b) return cout << a, b;
 
// debugger
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cout << name << " : " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
    const char* comma = strchr(names + 1, ',');cout.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}
#define debug(stuff) cout << #stuff << ": " << stuff <<endl
#define debugc(stuff) cout << #stuff << ": "; for(auto x: stuff) cout << x << " "; cout << endl;
// 
 
#define loop(i,a,b) for(ll i=a;i<b;i++)
#define all(v) v.begin(), v.end() 
 
#define print(stuff) cout << stuff << endl
#define printc(stuff) for(auto x: stuff) cout << x << " "; cout << endl;
#define printPrec(stuff) cout << fixed << setprecision(15) << stuff << endl;
#define len length
#define ret0 return 0
#define ret return 
 
 
#define ll long long
#define ld long double
#define fi first
#define endl '\n'
#define se second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define fill(ar, x) memset(ar, x, sizeof ar)
 
#define vl vector<ll> 
#define sl set<ll>
#define pll pair<ll, ll>
#define mll map<ll, ll> 
#define pq priority_queue<ll>
 
// typedef tree<ll,null_type,less<ll>,rb_tree_tag,
// tree_order_statistics_node_update> indexed_set;
 
#define inf (long long int) 1e18
#define eps 0.000001
#define mod 1000000007
#define mod1 998244353
#define MAXN (ll)1e5+5
#define N (1ll<<10)
#define pi 3.141592653589793238462643


ll fac[MAXN];

ll power(ll a, ll b, ll m){
    if (b==0)
        return 1;
    ll temp = power(a,b/2,m);
    temp = (temp*temp)%m;
    if (b&1)
        return (temp*(a%m))%m; // if b is odd a^b = a^(b/2)*a^(b/2)*a
    return temp;
}
  
// Returns n^(-1) mod p 
int modInverse(int n, int p) 
{ 
    return power(n, p-2, p); 
}

ll f(ll n, ll k, ll p){
    if(k == 0) return 1;
    // trace(n, k, p, fac[n], fac[k]);
    ll ans = fac[n];

    ans= (ans * modInverse(fac[k], p))%p;
    ans = (ans * modInverse(fac[n-k], p))%p;
    return ans;
}

void solve(){
    fac[0] = 1;
    // fac[1] = 1;
    for(ll i=1; i < MAXN; i++){
        fac[i] = (fac[i-1] * i)%mod; 
    }
    // trace(f(3, 1, mod));
    // trace(fac[1]);

    ll n, k;
    cin>>n>>k;
    ll a[n];
    loop(i,0,n){
        cin>>a[i];
    }
    sort(a, a+n);
    ll ans = 0;

    for(ll i=0; i<n; i++){
        ll val = 0;
        if(i >= k-1){
            // trace(i, k-1, mod, f(i, k-1, mod));
            val = (a[i] * f(i, k-1, mod))%mod;
        }
        ans = (ans + val)%mod;
        // trace(i, val);
    }
    loop(i,0,n){
        ll val = 0;
        if(n-1-i >= k-1){
            val = (a[i] * f(n-1-i, k-1, mod))%mod;
        }
        ans = (ans - val + mod)%mod;
    }
    print(ans);

}
 
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
 
    ll t = 1;
    // cin>>t;
    while(t--){
        solve();
    }
        

}


Measured memory_bytes: 1792