#include <stdio.h>
#include <algorithm>
using namespace std;
typedef long long ll;
ll a[100009], pre[100009] = {0};
const ll MOD = 1e9+7;
ll qpow(ll a, ll b)
{
	ll res = 1;
	while(b)
	{
		if(b%2) res = res * a % MOD;
		a = a * a % MOD;
		b /= 2;
	}
	return res;
}
int main()
{
	int n, k;
	scanf("%d %d", &n, &k);
	for(int i = 1; i <= n; i++)
		scanf("%lld", a+i);
	
	sort(a+1, a+1+n);
	for(int i = 1; i <= n; i++)
		pre[i] = (pre[i-1] + a[i]) % MOD;
		
	if(k==1) printf("0\n");
	else
	{
		ll res = 0, tmp = 1;
		for(int i = k, j = k-2; i <= n; i++, j++)
		{
			ll sum = (pre[n] - pre[i-1]  - pre[n-i+1] + MOD * 2) % MOD;
			res = (res + tmp * sum % MOD) % MOD;
			tmp = (tmp * (j + 1) % MOD) * qpow(j-k + 3, MOD-2) % MOD;
		}
			printf("%lld\n", res);
	}
	
 
}