#include<bits/stdc++.h>
using namespace std;
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 += "}";
    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 += "}";
    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)) + ")";
}

void debug_out() { cerr << endl; }

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

typedef long long ll;
typedef pair<int, int> pii;
#define mp make_pair
#define fst first
#define snd second
#define fr(i, a, b) for(int i=a; i<b; i++)
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;
template <typename T>
T inverse(T a, T m)
{
    T u = 0, v = 1;
    while (a != 0)
    {
        T t = m / a;
        m -= t * a;
        swap(a, m);
        u -= t * v;
        swap(u, v);
    }
    assert(m == 1);
    return u;
}
int main() {
    cin.tie(nullptr); ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    vector<int> vs(n);
    ll mod = 1e9+7;
    vector<ll> fact(n+1);
    fact[0] = fact[1] = 1;
    for(int i=2; i<=n; i++) fact[i] = fact[i-1] * i % mod;
    for(int i=0; i<n; i++) cin >> vs[i];
    sort(vs.begin(), vs.end());
    ll res = 0;
    for(int i=0; i<n-k+1; i++) { //c(i+1-2, k-2)
        res -= vs[i] * fact[n-i-1] %mod *inverse(fact[k-1], mod)%mod *inverse(fact[n-i-k], mod)%mod;
        res %= mod;
    }
    reverse(vs.begin(), vs.end());
    for(int i=0; i<n-k+1; i++) { //c(i+1-2, k-2)
        res += vs[i] * fact[n-i-1] %mod *inverse(fact[k-1], mod)%mod *inverse(fact[n-i-k], mod)%mod;
        res %=mod;
    }
    res = (res+mod)%mod;
    cout << res << endl;
}


Measured memory_bytes: 1408