#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;

const int maxn=100010,mod=1000000007;
typedef long long ll;
ll s[maxn],ans;
int n,k,a[maxn],inv[maxn],fac[maxn];

int c(int x,int y)
{
	return 1ll*fac[x]*inv[y]%mod*inv[x-y]%mod;
}

int main()
{
	int i,j;
	scanf("%d%d",&n,&k);
	if (k==1||n<k)
	{
		puts("0");
		return 0;
	}
	for (i=1;i<=n;i++)
		scanf("%d",&a[i]);
	sort(a+1,a+n+1);
	if (n==k)
	{
		printf("%d\n",(a[n]-a[1])%mod);
		return 0;
	}
	for (i=1;i<=n;i++)
		s[i]=s[i-1]+1ll*a[i];
	inv[0]=inv[1]=fac[0]=fac[1]=1;
	for (i=2;i<=n;i++)
		inv[i]=1ll*(mod-mod/i)*inv[mod%i]%mod;
	for (i=2;i<=n;i++)
	{
		inv[i]=1ll*inv[i-1]*inv[i]%mod;
		fac[i]=1ll*fac[i-1]*i%mod;
	}
	for (i=k-2;i<=n-2;i++)
	{
		ans=(ans+1ll*c(i,k-2)*(((s[n]-s[i+1]-s[n-i-1])%mod+mod)%mod)%mod)%mod;
	}
	printf("%d\n",ans);
	return 0;
}
