#include <iostream>
#include<bits/stdc++.h>
using namespace std;
long long a[100010];
long long fact[200020], finv[200020], inv[200020];
void facti() {
	fact[0] = fact[1] = 1;
	finv[0] = finv[1] = 1;
	inv[1] = 1;
	for (int i = 2; i < 200020; i++) {
		fact[i] = fact[i - 1] * i % 1000000007;
		inv[i] = 1000000007 - inv[1000000007 % i] * (1000000007 / i) % 1000000007;
		finv[i] = finv[i - 1] * inv[i] % 1000000007;
	}
}
long long ncr(int n, int k) {
	if (n < k) return 0;
	if (n < 0 || k < 0) return 0;
	return fact[n] * (finv[k] * finv[n - k] % 1000000007) % 1000000007;
}
int main() {
	long long n, k;
	cin >> n >> k;
	for (long long i = 0; i < n; i++) {
		cin >> a[i];
	}
	sort(a, a + n);
	facti();
	long long ans = 0;
	for (long long i = 0; i < n; i++) {
		ans = (ans - ncr(n - i - 1, k - 1) * a[i]) % 1000000007;
		ans = (ans + ncr(i, k - 1) * a[i]) % 1000000007;
	}
	cout << ans << endl;
	return 0;
}
