#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
const int p=(1e9)+7;
int c[100005][55];
void build(){
	c[0][0]=1;
	for(int i=1;i<=100000;i++){
		int k=min(50,i);
		c[i][0]=1;
		for(int j=1;j<=k;j++)
		c[i][j]=(c[i-1][j-1]+c[i-1][j])%p;
	}
}
int val[100005];
int main(){
	freopen("perica.in","r",stdin);
	freopen("perica.out","w",stdout); 
	build();
	int n,k;scanf("%d%d",&n,&k);
	for(int i=1;i<=n;i++)scanf("%d",&val[i]);
	sort(val+1,val+1+n);
	ll ans=0;
	for(int i=k;i<=n;i++)
	ans=(ans+(ll)val[i]*c[i-1][k-1])%p;
	printf("%lld\n",ans);
	return 0;
}