#include<cstdio> 
#include<cstring>
#include<iostream>
#include<algorithm>
#define maxn 100500
#define modu  1000000007LL
using namespace std;
typedef long long LL;
int n,k;
LL ans,a[maxn],c[maxn];
void gcd(LL a,LL b,LL &x,LL &y)
{
	if (!b)
	{
		x=1,y=0;
	} else
	{
		gcd(b,a%b,y,x);
		y-=(a/b)*x;
	}
}
int main()
{
	freopen("perica.in","r",stdin);
	freopen("perica.out","w",stdout);
	scanf("%d%d",&n,&k);
	for (int i=1;i<=n;++i)
		scanf("%d",&a[i]);
	sort(a+1,a+n+1);
	--k;
	c[k]=1;
	for (int i=k+1;i<=n;++i)
		gcd((LL)(i-k),modu,c[i],c[0]),
//		cout<<i<<" "<<c[i]<<" "<<(LL)(i-k)<<" "<<c[i]<<" "<<modu<<" "<<c[0]<<endl,
		c[i]=( (c[i]*c[i-1]) % modu *i) % modu,
		c[i]=( c[i]+ modu ) % modu;
//	for (int i=k;i<=n;++i)		cout<<c[i]<<endl;
	for (int i=n;i>=k;--i)
		ans+=(a[i]*c[i-1]) % modu,
		ans%=modu;
	cout<<ans<<endl;
	return 0;
}