//perica.cpp
#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

const int maxn=100100;
const int p=1000000007;
int n,k,a[maxn],u,v;
long long ans,t;
void ni(int x,int y){
	int u1,v1;
	if(y==0){
		u=1; v=0;
	}
	else{
		ni(y,x%y);
		u1=v; v1=u-x/y*v;
		u=u1; v=v1;
	}
}
int main(){
	freopen("perica.in","r",stdin);
	freopen("perica.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(int i=0;i<n;++i)scanf("%d",&a[i]);
	sort(a,a+n); ans=a[k-1]; t=1;
	for(int i=k;i<n;++i){
		ni(i-k+1,p);
		if(u<0)u+=p;
		t=t*i*u;
		ans=(ans+t*a[i])%p;
	}
	cout<<ans;
	return 0;
}