// Goods things are coming, just keep believing 
 
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
using namespace __gnu_pbds;
using namespace std;
 
 
#define FOR(i, x, n) for(ll i = x; i < n; i++) 
#define pb push_back
#define pf push_front
#define ll unsigned long long
#define hii cout << "hii" << endl
#define okay cout << "okay" << endl
// #define hey cout << "hey" << endl
#define pii pair<int, int>
#define pll pair<ll, ll>
// #define mpp make_pair
#define endl '\n'
#define ff first 
#define int long long
#define ss second
#define vi vector<int>
#define all(s) s.begin(), s.end()
#define si size()
 #define ld long double
#define INF 1000000000;
 
template <class T> ostream& operator << (ostream &os, const vector<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T> ostream& operator << (ostream &os, const set<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T, class S> ostream& operator << (ostream &os, const pair<T, S> &v) { os << v.first << ' ' << v.second; return os; }
template <class T, class S> ostream& operator << (ostream &os, const unordered_map<T, S> &v) { for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' '; return os; } 
 
 
 
#ifndef ONLINE_JUDGE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template <class Arg1> void __f(const char* name, Arg1&& arg1) { cerr << name << " : " << arg1 << endl; }
    template <class Arg1, class... Args>
    void __f(const char* names, Arg1&& arg1, Args&&... args) {
        const char* sep = strchr(names + 1, ',');
        cerr.write(names, sep - names) << " : " << arg1 << "  ";
        __f(sep + 1, args...);
    }
#else
#define trace(...) 0
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("avx2,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define _CRT_SECURE_NO_WARNINGS
#endif // ifndef ONLINE_JUDGE
 
typedef tree<
int,
null_type,
less<int>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
 
 
 
const int N =1e5 + 5;
const int LG = 20;
const int mod = 1e9 + 7;
const int MOD = 1e9 + 7;

int arr[N];
int fact[N], invfact[N];

int pow(int a, int b, int m)
{
	int ans=1;
	while(b)
	{
		if(b&1)
			ans=(ans*a)%m;
		b/=2;
		a=(a*a)%m;
	}
	return ans;
}

int modinv(int k)
{
	return pow(k, MOD-2, MOD);
}

void precompute()
{
	fact[0]=fact[1]=1;
	for(int i=2;i<N;i++)
	{
		fact[i]=fact[i-1]*i;
		fact[i]%=MOD;
	}
	invfact[N-1]=modinv(fact[N-1]);
	for(int i=N-2;i>=0;i--)
	{
		invfact[i]=invfact[i+1]*(i+1);
		invfact[i]%=MOD;
	}
}

int nCr(int x, int y)
{
	if(y>x)
		return 0;
	int num=fact[x];
	num*=invfact[y];
	num%=MOD;
	num*=invfact[x-y];
	num%=MOD;
	return num;
}

void solve()
{
	int n, k;
	cin >> n >> k;
	precompute();
	for(int i = 0; i < n; i++)
	{
		cin >> arr[i];
	}
	sort(arr, arr + n);
	int mn1 = 0;
	int mn2 = 0;
	int mx1 = 0;
	int mx2 = 0;
	int right = n;
	for(int i = 0; i < n; i++)
	{
		right--;
		int temp = nCr(right, k - 1);
		if(arr[i] < 0)
		{
		    int el = -arr[i];
		    mn1 = ((temp * el) % mod + mn1) % mod;
		}
		else
		{
		    int el = arr[i];
		    mn2 = ((temp * el) % mod + mn2) % mod;
		}
	}
	right = n;
	for(int i = n - 1; i >= 0; i--)
	{
		right--;
		int temp = nCr(right, k - 1);
		if(arr[i] < 0)
		{
		    int el = -arr[i];
		    mx1 = ((temp * el) % mod + mx1) % mod;
		}
		else
		{
		    int el = arr[i];
		    mx2 = ((temp * el) % mod + mx2) % mod;
		}
	}
	int val1 = (mn1 + mx2) % mod;
	int val2 = (mn2 + mx1) % mod;
	cout << (val1 - val2 + mod) % mod << endl;
}
 
int32_t main()
{
	ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
	int t = 1;
	// cin >> t;
	while(t--)
	{
		solve();
	}
}


Measured memory_bytes: 2560