#include <bits/stdc++.h>
#define int long long
#define INF 1000000007ll
using namespace std;

int n,k;
int a[100010],pw[100010],rev[100010];

inline int my_pow(int x,int y)
{
  if(y==0) return 1;
  if(y==1) return x;
  int res=my_pow(x,y/2);
  if(y%2==0) return (res*res)%INF;
  else return (((res*res)%INF)*x)%INF;
}

signed main()
{
  ios::sync_with_stdio(false);
  cin>>n>>k;
  for(int i=1;i<=n;i++) cin>>a[i];
  sort(a+1,a+n+1);
  pw[0]=1;rev[0]=1;
  for(int i=1;i<=n;i++){
    pw[i]=(pw[i-1]*i)%INF;
    rev[i]=my_pow(pw[i],INF-2ll);
  }
  int ans=0ll;
  for(int i=1;i<=n-k+1;i++){
    int val=pw[n-i]*rev[n-i-k+1];
    val%=INF;
    val*=rev[k-1];val%=INF;
    val*=a[i];val%=INF;
    ans-=val;
  }
  for(int i=k;i<=n;i++){
    int val=pw[i-1]*rev[i-k];
    val%=INF;
    val*=rev[k-1];val%=INF;
    val*=a[i];val%=INF;
    ans+=val;
  }
  ans%=INF;
  if(ans<0) ans+=INF;
  cout<<ans<<endl;
  return 0;
}