// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>
#include <fstream>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest& ...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest& ...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest& ...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)std::cin>>w;
// output
template<typename T>void MACRO_OUT(const T t) { std::cout << t; }
template<typename First, typename...Rest>void MACRO_OUT(const First first, const Rest...rest) { std::cout << first << " "; MACRO_OUT(rest...); }
#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define FOUT(n, dist) std::cout<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) std::cout<<std::setw(n)<<std::setfill(c)<<(dist);
#define EOUT(...) { OUT(__VA_ARGS__)BR; exit(0); }
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t: " << (dist) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t: ";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
#define SHOWSTACK(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.top() << " ";tmp.pop();}std::cerr << "\n";}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define REP(w, n) for(int w=0;w<int(n);++w)
#define RREP(w, n) for(int w=int(n)-1;w>=0;--w)
#define IN(a, x, b) (a<=x && x<b)
template<class T> inline T CHMAX(T & a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                    // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-10;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T> & a, const T & val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>> & a, const T & val) { for (auto& b : a) FILL(b, val); }
// ------------>8------------------------------------->8------------

ll powMod(ll n, ll p, ll mod) {
	ll res = 1;
	while (p) {
		if (p & 1) (res *= n) %= mod;
		(n *= n) %= mod;
		p >>= 1;
	}
	return res;
}
ll invMod(ll n, ll mod) {
	return powMod(n, MOD - 2, MOD);
}

const signed FACT_MAX_N = 1000006;
signed fact[FACT_MAX_N];
signed factInv[FACT_MAX_N];
struct INIT_FACT {
	INIT_FACT() {
		fact[0] = 1;
		for (int i = 1; i < FACT_MAX_N; ++i) fact[i] = (long long)i * fact[i - 1] % MOD;
		factInv[FACT_MAX_N - 1] = powMod(fact[FACT_MAX_N - 1], MOD - 2, MOD);
		for (int i = FACT_MAX_N - 2; i >= 0; --i) factInv[i] = (long long)(i + 1) * factInv[i + 1] % MOD;
	}
} init_fact;

#if true // true if use MOD
/* n,r<=10^6, query O(1)*/
ll Permutation(int n, int r) {
	if (r == 0) return 1;
	if (n < r) return 0;
	return (long long)fact[n] * factInv[n - r] % MOD;
}
ll Combination(int n, int r) {
	if (r == 0) return 1;
	if (n < r) return 0;
	return (long long)fact[n] * factInv[n - r] % MOD * factInv[r] % MOD;
}
ll CombinationWithRepetition(ll n, ll r) {
	return Combination(n + r - 1, r);
}
/*n<=10^9, r<=10^5, query O(r)*/
/*ll Combination(ll n, ll r, ll mod) {
	if (n < r) return 0;
	ll ans = 1;
	if (n < 2 * r) r = n - r;
	for (int i = 1; i <= r; ++i) {
		ans *= n - i + 1; ans %= mod;
		ans *= powMod(i, mod - 2, mod); ans %= mod;
	}
	return ans;
}
ll Permutation(int n, int r, int mod) {
	return Combination(n, r, mod) * fact[r] % MOD;
}*/
#else // unuse MOD
const signed COMB_MAX_N = 67; // <- max : 67
long long comb[COMB_MAX_N][COMB_MAX_N];
struct INIT_COMB {
	INIT_COMB() {
		for (int i = 0; i < COMB_MAX_N; ++i) {
			for (int j = 0; j <= i; ++j) {
				if (j == 0 || j == i) comb[i][j] = 1;
				else comb[i][j] = comb[i - 1][j - 1] + comb[i - 1][j];
			}
		}
	}
} init_comb;
#endif

signed main() {
	INIT;

	VAR(int, n, k);
	VEC(int, a, n);

	std::sort(ALL(a));
	int ans = 0;
	REP(i, n) {
		
		(ans += a[i] * Combination(i, k - 1)) %= MOD;
      (ans -= a[i] * Combination(n - 1 - i, k - 1)) %= MOD;
	}
	if (ans < 0) ans += MOD;
	OUT(ans)BR;

	return 0;
}


Measured memory_bytes: 8832