#pragma GCC optimize "O3"
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

#define POP pop_back
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define F first
#define S second
#define X real()
#define Y imag()
#ifndef LOCAL
    #define endl "\n"
#endif

using namespace std;
using namespace chrono;
using namespace __gnu_pbds;

typedef pair<int, int> ii;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> pll;
typedef long long C;
typedef complex<C> P;

const int INF = 1e9;
const long long LLINF = 1e18;
const double PI = 3.14159265358979323;
const long long PRIME = (1ll << 31) - 1;
const long long PRIME2 = 269;
const long long MOD = 1e9 + 7;

template <class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <class T, class U> using ordered_map = tree<T, U, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <class T> using hash_set = gp_hash_table<T, null_type>;
template <class T, class U> using hash_map = gp_hash_table<T, U>;
template <class T, class U> ostream& operator<< (ostream &out, pair<T, U> p) { return out << "(" << p.F << ", " << p.S << ")"; }
template <class T> ostream& operator<< (ostream &out, vector<T> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T> ostream& operator<< (ostream &out, set<T> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T, class U> ostream& operator<< (ostream &out, map<T, U> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T> ostream& operator<< (ostream &out, ordered_set<T> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T, class U> ostream& operator<< (ostream &out, ordered_map<T, U> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T> ostream& operator<< (ostream &out, hash_set<T> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T, class U> ostream& operator<< (ostream &out, hash_map<T, U> c) { for (auto it = c.begin(); it != c.end(); ++it) { if (it != c.begin()) out << " "; out << *it; } return out; }
template <class T> int sign (T x) { return (x > 0) - (x < 0); }
inline ll mod (ll a, ll b) { return a%b + (a < 0) * b; }
inline ll mod (ll a) { return mod(a, MOD); }
C dot (P a, P b) { return a.X * b.X + a.Y * b.Y; }
C cross (P a, P b) { return a.X * b.Y - b.X * a.Y; }

double startTime;
double currTime() { return duration_cast<nanoseconds>(system_clock::now().time_since_epoch()).count() / 1e9; }
double execTime() { return currTime() - startTime; }

vector<ll> fac;
ll factorial (int n, ll modulo) {
    while (fac.size() <= n) {
        if (fac.size() == 0)
            fac.PB(mod(1, modulo));
        else
            fac.PB(mod(fac.size() * fac[fac.size() - 1], modulo));
    }
    return fac[n];
}

ll multiply (ll a, ll b, ll modulo) { a = mod(a, modulo); b = mod(b, modulo); ll result = 0; while (b > 0) { if (b & 1) result = mod(result + a, modulo); b = b >> 1; a = mod(a * 2, modulo); } return result; }
ll pow (ll base, ll exp, ll modulo) { if (exp == 0) return mod(1, modulo); ll result = mod(1, modulo); base = mod(base, modulo); while (exp > 0) { if (exp & 1) result = multiply(result, base, modulo); exp = exp >> 1; base = multiply(base, base, modulo); } return result; }
ll inv (ll a, ll modulo) { return pow(a, modulo - 2, modulo); }
ll gcd (ll a, ll b, ll *x = NULL, ll *y = NULL) { a = abs(a), b = abs(b); pll s = { 1, 0 }, t = { 0, 1 }, r = { a, b }; while (r.S != 0) { ll q = r.F / r.S; r = { r.S, r.F - q * r.S }; s = { s.S, s.F - q * s.S }; t = { t.S, t.F - q * t.S }; } if (x) *x = s.F; if (y) *y = t.F; return r.F; }
vector<int> spf; void build_sieve (int n) { spf.resize(n + 1); spf[0] = spf[1] = -1; for (int i = 2; i <= n; i++) spf[i] = i; for (int i = 2; i <= n; i++) if (spf[i] == i) for (int j = 2 * i; j <= n; j += i) spf[j] = min(spf[j], i); }
bool miller_test (ll n, ll d, ll r, ll a) { ll x = pow(a, d, n); if (x == 1 || x == n - 1) return true; for (int i = 0; i < r; i++) { x = multiply(x, x, n); if (x == n - 1) return true; } return false; }
bool is_prime (ll n) { if (n < 2) return false; if (n < 4) return true; ll d = n - 1, r = 0; while (d % 2 == 0) { d /= 2; r++; } vector<ll> a{ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37 }; for (int i = 0; i < a.size() && a[i] <= n - 2; i++) if (!miller_test(n, d, r, a[i])) return false; return true; }
ll pollard_rho (ll n) { for (ll a = 1; a <= n - 1; a++) { auto f = [&](ll x) { return mod(multiply(x, x, n) + a, n); }; for (ll b = 0; b <= n - 1; b++) { ll x = b, y = b; do { x = f(x); y = f(f(y)); ll p = gcd(abs(x - y), n); if (p > 1 && p < n) return p; } while (x != y); } } return 0; }
vector<pll> factorise (ll n) { if (n < 2) return vector<pll>(); if (n <= spf.size()) { vector<pll> ans; while (n > 1) { if (ans.empty() || ans.back().F != spf[n]) ans.PB(MP(spf[n], 1)); else ans.back().S++; n /= spf[n]; } return ans; } else { vector<ll> v, factors; v.PB(n); while (!v.empty()) { ll x = v.back(); v.POP(); if (is_prime(x)) factors.PB(x); else if (x % 2 == 0) { v.PB(2); v.PB(x/2); } else { ll p = pollard_rho(x), q = x/p; v.PB(p); v.PB(q); } } sort(factors.begin(), factors.end()); vector<pll> ans; for (int i = 0; i < factors.size(); i++) if (ans.empty() || ans.back().F != factors[i]) ans.PB(MP(factors[i], 1)); else ans.back().S++; return ans; } }

ll n_choose_k (int n, int k, ll modulo) {
    return multiply(factorial(n, modulo), inv(multiply(factorial(k, modulo), factorial(n - k, modulo), modulo), modulo), modulo);
}

int main() {
#ifndef LOCAL
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
#endif
    srand(time(0));
    startTime = currTime();

    int n, k;
    cin >> n >> k;

    vector<ll> v(n);
    for (int i = 0; i < n; i++)
        cin >> v[i];
    sort(v.begin(), v.end());

    ll sum = 0;
    for (int i = k - 1; i < n; i++)
        sum = mod(sum + multiply(n_choose_k(i, k - 1, MOD), v[i], MOD), MOD);
    reverse(v.begin(), v.end());
    for (int i = k - 1; i < n; i++)
        sum = mod(sum - multiply(n_choose_k(i, k - 1, MOD), v[i], MOD), MOD);
    cout << sum << endl;
}

Measured memory_bytes: 2296