#include <iostream>
#include <cassert>
#include <vector>
#include <algorithm>

template<int M>
struct ModInt {
public:
	using value_type = long long;
	
	ModInt(value_type val = 0) : val(val < 0 ? (M - (-val % M)) % M : val % M) {}
	
	bool operator ==(const ModInt & rhs) const noexcept { return val == rhs.val; }
	bool operator !=(const ModInt & rhs) const noexcept { return !(*this == rhs); }
	ModInt operator +() const noexcept { return ModInt(*this); }
	ModInt operator -() const noexcept { return ModInt(0) -= *this; }
	ModInt operator +(const ModInt & rhs) const noexcept { return ModInt(*this) += rhs; }
	ModInt operator -(const ModInt & rhs) const noexcept { return ModInt(*this) -= rhs; }
	ModInt operator *(const ModInt & rhs) const noexcept { return ModInt(*this) *= rhs; }
	ModInt operator /(const ModInt & rhs) const noexcept { return ModInt(*this) /= rhs; }
	
	ModInt & operator +=(const ModInt & rhs) noexcept {
		val += rhs.val;
		if (val >= M) val -= M;
		return *this;
	}
	
	ModInt & operator -=(const ModInt & rhs) noexcept {
		if (val < rhs.val) val += M;
		val -= rhs.val;
		return *this;
	}
	
	ModInt & operator *=(const ModInt & rhs) noexcept {
		val = val * rhs.val % M;
		return *this;
	}
	
	ModInt & operator /=(const ModInt & rhs) noexcept {
		*this *= rhs.invese();
		return *this;
	}
	
	ModInt pow(value_type n) const {
		ModInt res = 1, x = val;
		if (n < 0) {
			x = ModInt(x).inverse();
			n = -n;
		}
		while (n) {
			if (n & 1) res *= x;
			x *= x;
			n >>= 1;
		}
		return res;
	}
	
	ModInt inverse() const {
		value_type a = val, a1 = 1, a2 = 0, b = M, b1 = 0, b2 = 1;
		while (b > 0) {
			value_type q = a / b;
			int r = a % b;
			int nb1 = a1 - q * b1;
			int nb2 = a2 - q * b2;
			a = b; b = r;
			a1 = b1; b1 = nb1;
			a2 = b2; b2 = nb2;
		}
		assert(a == 1);
		return a1;
	}
	
	const value_type & get() const noexcept { return val; }
	value_type & get() noexcept { return val; }
	static decltype(M) get_mod() noexcept { return M; }
	
	friend std::ostream & operator <<(std::ostream & os, const ModInt & rhs) { return os << rhs.val; }
	friend std::istream & operator >>(std::istream & is, ModInt & rhs) {
		value_type x;
		is >> x;
		rhs = ModInt(x);
		return is;
	}
private:
	value_type val;
};

template<typename T>
struct Combination {
public:
	using size_type = std::size_t;
	
	Combination(size_type sz = 1) : _fact(1, 1), _finv(1, 1), _inv(1, 1) { build(sz); }
	
	T fact(size_type k) { if (k >= T::get_mod()) return 0; build(k); return _fact[k]; }
	T finv(size_type k) { assert(k < T::get_mod()); build(k); return _finv[k]; }
	T inv(size_type k) { assert(k > 0 && k < T::get_mod()); build(k); return _inv[k]; }
	
	T operator ()(int n, int r) { return c(n, r); }
	T c(int n, int r) {
		if (r < 0 || n < r) return 0;
		return fact(n) * finv(r) * finv(n - r);
	}
	
private:
	std::vector<T> _fact, _finv, _inv;
	static constexpr size_type MAX_LIMIT = 50000000;
	
	void build(size_type k) {
		if (_fact.size() > k) return;
		assert(k < MAX_LIMIT);
		size_type sz = std::min({MAX_LIMIT, static_cast<size_type>(T::get_mod()), std::max(_fact.size() * 2, k + 1)});
		size_type presz = _fact.size();
		_fact.resize(sz);
		_finv.resize(sz);
		_inv.resize(sz);
		
		for (size_type i = presz; i < sz; ++i) _fact[i] = _fact[i - 1] * i;
		_finv[sz - 1] = T(_fact[sz - 1]).inverse();
		for (size_type i = sz - 1; i > presz; --i) {
			_finv[i - 1] = _finv[i] * i;
			_inv[i] = _fact[i - 1] * _finv[i];
		}
		_inv[presz] = _fact[presz - 1] * _finv[presz];
	}
};

int main() {
	int N, K;
	std::cin >> N >> K;
	
	std::vector<int> A(N);
	for (int i = 0; i < N; ++i) scanf("%d", &A[i]);
	
	constexpr int MOD = 1000000007;
	ModInt<MOD> ans;
	Combination<ModInt<MOD>> comb;
	
	std::sort(A.begin(), A.end());
	for (int t = 0; t < 2; ++t) {
		for (int i = N - 1; i >= 0; i--) {
			ans += comb(i, K - 1) * (t ? -A[i] : A[i]);
		}
		std::reverse(A.begin(), A.end());
	}
	std::cout << ans << std::endl;
	return 0;
}

Measured memory_bytes: 6088