#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp>  
#include <functional>
#include <iostream> 
#include<bits/stdc++.h>
using namespace __gnu_pbds;  
using namespace std;  
// Declaring ordered_set 
typedef tree<long long int, null_type, less<long long int>, rb_tree_tag,  
            tree_order_statistics_node_update>  
    ordered_set;  
typedef pair<long long int,long long int> pa;
#define resize(A) A.resize(unique(A.begin(), A.end()) - A.begin())
#define pb push_back
#define  MAX 1000005
long long int MOD=1e9+7;
long long int INF=1e18+4;

///////////////////////////////////////////////sieve////////////////////////////////////////////////////////////////////
 /* bool B[MAX];
vector<long long int>V1;
 void sieve()
 {
    long long int i,j;
    for(i=2;i<MAX;i++)
    {
      if(!B[i])
     { 
          V1.pb(i);
        for(j=2*i;j<MAX;j=j+i)
             B[j]=true;
      }
    }
 }
 */

 
 
//////////////////////////////////////////////////////////D-S-U/////////////////////////////////////////////////////////
/*long long int parent[MAX];
long long int size[MAX];
void initialise()
{ //cout<<MAX<<" ";
    for(long  long int i=0;i<MAX;i++)
    {
        parent[i]=i;
        //cout<<i<<" ";
        size[i]=1;
    }
}
long long int  root(long long int x)
{
    while(x!=parent[x])
    {
        parent[x]=parent[parent[x]];
        x=parent[x];
    }
    return x;
}
 
    
void union1(long long int x,long long int y)
{
    if(size[x]>=size[y])
    {
        size[x]+=size[y];
        parent[y]=x;
        
    }
    else
    {
        size[y]+=size[x];
        parent[x]=y;
    }
}*/
//////////////////////////////////////////////////////MODPOWER//////////////////////////////////////////////////////////

 
long long int modulo(long long int a,long long  int b, long long int n){
    long long int  x=1, y=a%n; 
    while (b > 0) {
        if (b%2 == 1) {
            x = (x*y) % n; // multiplying with base
        }
        y = (y*y) % n; // squaring the base
        b /= 2;
    }
    return x % n;
}
 
 
 
////////////////////////////////////////////////BIT/////////////////////////////////////////////////////////////////////
/*long long int sum(long long int bit[],long long int in,long long int n)
{
    //in=in+1;
    long long int sum=0;
    if(in==0)
        return 0;
    while(in>0)
    {
      sum=sum+bit[in];
        
        in=in-(in&(-in));
    }
    return sum;
}
void update(long long int bit[],long long int in,long long int n,long long int val)
{
    // in=in+1;
    
    while(in<=n)
    {
       // cout<<in<<" ";
        bit[in]+=val;
        in=in+ (in &(-in));
    }
}*/
long long int R[4]={1,-1,0,0};
long long int C[4]={0,0,1,-1};
int main()
{
ios_base::sync_with_stdio(false); 
cin.tie(NULL);
cout.tie(NULL);
long long int d,i,w,j,k=0,l=0,r=0,m=0,n,c,p=0,cnt=0,cnt1=0,x,y,z=0,a,b,low,mid,high,ans=0,maxi=0,mini=INF,t1,x1,y1,t,sum=0,i1,j1;
string s,s1;    long long qt,xx;        char jj;
    
    map<long long int,long long int >ma;
    vector<long long int>V;
   cin>>n>>k;
    long long  int fact[n];
    fact[0]=1;
    for(i=1;i<n;i++)
    {
        fact[i]=(fact[i-1]*i)%MOD;
    }
    long long int A[n];
    for(i=0;i<n;i++)
    {
        cin>>A[i];
    }
    sort(A,A+n);
    for(i=0;i<n;i++)
    {
        V.pb(A[i]);
        ma[A[i]]++;
    }
    //resize(V);
    if(k==1)
    {
        cout<<0;
        return 0;
    }
    l=0;
    ans=0;
    for(i=0;i<V.size();i++)
    {
         cnt=1;
        if(l>=k-1)
        {
            p=(fact[k-1]*fact[l-k+1])%MOD;
            p=modulo(p,MOD-2,MOD);
            p=(p*fact[l])%MOD;
            p=(cnt*p)%MOD;
            //cerr<<p<<" 
            p=(p*V[i])%MOD;
            cerr<<p<<" ";
            ans=(ans+p+MOD)%MOD;
        }
       
        l++;
        if(n-l>=k-1)
        {
            p=(fact[k-1]*fact[n-l-k+1])%MOD;
            p=modulo(p,MOD-2,MOD);
            p=(p*fact[n-l])%MOD;
           p=(cnt*p)%MOD;
             p=(p*V[i])%MOD;
             cerr<<p<<" ";
            ans=(ans-p+MOD)%MOD;
            
        }
       // l=l+r-1;
            
        
    } cout<<ans<<endl;
    

 
}
    

    
    
    


Measured memory_bytes: 8308