/*

  "With age, comes wisdom. With travel, comes understanding.
   Remember that happiness is a way of travel – not a destination"

*/

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define int            long long int
#define ld             long double
#define F              first
#define S              second
#define pb             push_back
#define si             set <int>
#define vi             vector <int>
#define pii            pair <int, int>
#define vpi            vector <pii>
#define vpp            vector <pair<int, pii>>
#define mii            map <int, int>
#define mpi            map <pii, int>
#define spi            set <pii>
#define endl           "\n"
#define sz(x)          ((int) x.size())
#define all(p)         p.begin(), p.end()
#define que_max        priority_queue <int>
#define que_min        priority_queue <int, vi, greater<int>>
#define bug(...)       __f (#__VA_ARGS__, __VA_ARGS__)
#define print(a)       for(auto x : a) cout << x << " "; cout << endl
#define print1(a)      for(auto x : a) cout << x.F << " " << x.S << endl
#define print2(a,x,y)  for(int i = x; i < y; i++) cout<< a[i]<< " "; cout << endl

inline int power(int a, int b)
{
	int x = 1;
	while (b)
	{
		if (b & 1) x *= a;
		a *= a;
		b >>= 1;
	}
	return x;
}

typedef tree<pii, null_type, less<pii>, rb_tree_tag, tree_order_statistics_node_update> Set;

template <typename Arg1>
void __f (const char* name, Arg1&& arg1) { cout << name << " : " << arg1 << endl; }
template <typename Arg1, typename... Args>
void __f (const char* names, Arg1&& arg1, Args&&... args)
{
	const char* comma = strchr (names + 1, ',');
	cout.write (names, comma - names) << " : " << arg1 << " | "; __f (comma + 1, args...);
}

const int N = 200005;
int a[N], n, m, k;


int fact[N], mod = 1e9 + 7;
inline int add(int x, int y) { x += y; if (x >= mod) x -= mod; return x; }
inline int sub(int x, int y) { x -= y; if (x < 0) x += mod; return x; }
inline int mul(int x, int y) { return ((x % mod) * (y % mod)) % mod; }
inline int powr(int a, int b) {
	int x = 1 % mod;
	while (b) {
		if (b & 1) x = mul(x, a);
		a = mul(a, a);
		b >>= 1;
	}
	return x;
}
inline int inv(int a) { return powr(a, mod - 2); }

void precompute()
{
	fact[0] = 1;
	for (int i = 1; i < N; i++)
		fact[i] = mul(fact[i - 1], i);
}
int nCr(int num, int den)
{
	int den1 = fact[den], den2 = fact[num - den];
	num = fact[num];
	den = mul(den1, den2);
	int ans = mul(num, inv(den));
	return ans;
}


void solve()
{
	int i, j;
	cin >> n >> k;

	precompute();
	for (i = 1; i <= n; i++)
		cin >> a[i];

	sort(a + 1, a + n + 1);
	for (i = 1; i <= n; i++)
		if (a[i] < 0)
			a[i] += mod;

	int ans = 0;
	for (i = k; i <= n; i++)
	{
		int x = mul(a[i], nCr(i - 1, k - 1));
		ans = add(ans, x);
	}
	for (i = n - k + 1; i >= 1; i--)
	{
		int x = mul(a[i], nCr(n - i, k - 1));
		ans = sub(ans, x);
	}
	cout << ans;


}

int32_t main()
{
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

	cout << setprecision(9) << fixed;
	clock_t z = clock();
	solve();
	cerr << "Run Time : " << ((double)(clock() - z) / CLOCKS_PER_SEC);

	return 0;
}

Measured memory_bytes: 2560