#define LOCAL

#undef _GLIBCXX_DEBUG

#include <bits/stdc++.h>
using namespace std;

#define IOS ios::sync_with_stdio(false);cin.tie(0)
#define all(x) x.begin(), x.end()
#define ff first
#define ss second
#define LLINF 1e18
#define INF (int)1e9+1

// Copied from Gennady-Korotkevich's template

template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
	return '"' + s + '"';
}

string to_string(const char* s) {
	return to_string((string)s);
}

string to_string(bool b) {
	return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
	bool first = true;
	string res = "{";
	for (int i = 0; i < static_cast<int>(v.size()); i++) {
		if (!first) {
			res += ", ";
		}
		first = false;
		res += to_string(v[i]);
	}
	res += "}\n";
	return res;
}

template <size_t N>
string to_string(bitset<N> v) {
	string res = "";
	for (size_t i = 0; i < N; i++) {
		res += static_cast<char>('0' + v[i]);
	}
	return res;
}

template <typename A>
string to_string(A v) {
	bool first = true;
	string res = "{";
	for (const auto& x : v) {
		if (!first) {
			res += ", ";
		}
		first = false;
		res += to_string(x);
	}
	res += "}\n";
	return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
	return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
	return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
	return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

template <typename A, typename B, typename C, typename D, typename E>
string to_string(tuple<A, B, C, D, E> p) {
	return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + "," + to_string(get<4>(p)) + ")";
}

void debug_out() {
	cerr << endl;
}

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
	cerr << " " << to_string(H);
	debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "\n[" << #__VA_ARGS__ << "]:\n", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

// End of Gennady-Korotkevich's template 

using llong = long long;
using PII = pair<int, int>;

const llong MOD = 1e9 + 7;
const double PI = acos(-1);

const int N = 200200;
int f[N], rf[N];

llong add(llong a, llong b) {
	a += b;
	return a % MOD;
}
llong sub(llong a, llong b) {
	a -= b; a %= MOD;
	if (a < 0) return a + MOD;
	return a;
}
llong mult(llong a, llong b) {
	return (a * b) % MOD;
}
llong bin_pow(llong a, llong p) {
	if (p == 0) return 1;
	if (p & 1) return mult(a, bin_pow(a, p - 1));
	return bin_pow(mult(a, a), p / 2);
}
llong rev(llong a) {
	return bin_pow(a, MOD - 2);
}
llong getC(llong n, llong k) {
	if (k < 0 || k > n) return 0;
	return mult(f[n], mult(rf[n - k], rf[k]));
}
int main() {
	IOS;
	int n, k; cin >> n >> k;
	f[0] = 1;
	for (int i = 1; i < N; i++) {
		f[i] = mult(f[i - 1], i);
	}
	rf[N - 1] = rev(f[N - 1]);
	for (int i = N - 1; i > 0; i--) {
		rf[i - 1] = mult(rf[i], i);
	}
	vector<llong> a(n);
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	llong ans = 0;
	sort(a.rbegin(), a.rend());
	for (int i = 0; i < n; i++) {
		ans = add(ans, a[i] * getC(n - i - 1, k - 1));
	}
	sort(a.begin(), a.end());
	for (int i = 0; i < n; i++) {
		ans = sub(ans, a[i] * getC(n - i - 1, k - 1));
	}
	cout << ans;
}

Measured memory_bytes: 5560