#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#define rep(i, start, end) for (int i = (int)start; i < (int)end; ++i)
#define rrep(i, start, end) for (int i = (int)start - 1; i >= end; --i)
#define iter(i, c) for (auto i = (c).begin(); i != (c).end(); ++i)
#define riter(i, c) for (auto i = (c).rbegin(); i != (c).rend(); ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using namespace std;
using ll = long long;

template<typename T, long long MOD_VALUE>
class ModInt {
    static constexpr long long MOD = MOD_VALUE;
    private:
        T value_;
    public:
        ModInt() {}
        ModInt(const T& value):value_((value + MOD) % MOD) {}
        ModInt& operator+=(const ModInt& x)  {value_ += x.value_; if (value_ >= MOD) value_ -= MOD; return *this;}
        friend ModInt& operator+=(const T& x, const ModInt& y) {ModInt res(x); res.value_ += x.value_; if (res.value_ >= MOD) res.value_ -= MOD; return res;}
        ModInt& operator-=(const ModInt& x) {if (value_ < x.value_) value_ += MOD; value_ -= x.value_; return *this;}
        friend ModInt& operator-=(const T& x, const ModInt& y) {ModInt res(x); if (res.value_ < y.value_) res.value_ += MOD; res.value_ -= y.value_; return res;}
        ModInt& operator*=(const ModInt& x) {value_ = (value_ * x.value_) % MOD; return *this;}
        friend ModInt& operator*=(const T& x, const ModInt& y) {ModInt res(x); res.value_ = (res.value_ * y.value_) % MOD; return res;}
        const ModInt operator+(const ModInt& x) const {return ModInt(*this) += x;}
        friend const ModInt operator+(const T& x, const ModInt& y) {return ModInt(x) += y;}
        const ModInt operator-(const ModInt& x) const {return ModInt(*this) -= x;}
        friend const ModInt operator-(const T& x, const ModInt& y) {return ModInt(x) -= y;}
        const ModInt operator*(const ModInt& x) const {return ModInt(*this) *= x;}
        friend const ModInt operator*(const T& x, const ModInt& y) {return ModInt(x) *= y;}
        static ModInt modpow(ModInt x, long long y) {ModInt z = 1; while (y > 0) {if (y & 1) {z *= x;}x *= x; y /= 2;} return z;}
        ModInt& operator/=(const ModInt& x) {return *this *= modpow(x, MOD - 2);}
        const ModInt operator/(const ModInt& x) const {return ModInt(*this) /= x;}
        friend const ModInt operator/(const T& x, const ModInt& y) {return ModInt(x) /= y;}
        ModInt operator++(int) {ModInt tmp(*this); value_ = (value_ + 1 == MOD ? 0 : value_ + 1); return tmp;}
        ModInt operator--(int) {ModInt tmp(*this); value_ = (value_ - 1 < 0 ? MOD - 1 : value_ - 1); return tmp;}
        friend istream& operator>>(istream& stream, ModInt& x) {stream >> x.value_; x.value_ %= MOD; return stream;}
        friend ostream& operator<<(ostream& stream, const ModInt& x) {stream << x.value_; return stream;}
};

template<typename T, long long MOD>
class Combination {
    using mint = ModInt<T, MOD>;
    private:
        vector<mint> factorial_, inv_factorial_, inv_;
    public:
        Combination(int max_num)
            :factorial_(max_num + 1), inv_factorial_(max_num + 1), inv_(max_num + 1) {
            factorial_[0] = factorial_[1] = 1;
            inv_factorial_[0] = inv_factorial_[1] = 1;
            inv_[1] = 1;
            for (int i = 2; i <= max_num; i++) {
                factorial_[i] = factorial_[i - 1] * i;
                inv_[i] = MOD - inv_[MOD % i] * (MOD / i);
                inv_factorial_[i] = inv_factorial_[i - 1] * inv_[i];
            }
        }
        mint calc(int n, int k) {
            if (n < k) return 0;
            if (n < 0 || k < 0) return 0;
            return factorial_[n] * inv_factorial_[k] * inv_factorial_[n - k];
        }
};

using mint = ModInt<ll, 1000000007>;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N, K;
    cin >> N >> K;
    vector<ll> A(N);
    rep(i, 0, N) cin >> A[i];
    if (N == 1 || K == 1) {
        cout << 0 << endl;
        return 0;
    }
    sort(all(A));
    reverse(all(A));
    Combination<ll, 1000000007> comb(N);
    vector<mint> cumsum(N + 1, 0);
    rep(i, K - 2, N + 1) {
        cumsum[i] = comb.calc(i, K - 2);
        if (i - 1 >= 0) cumsum[i] += cumsum[i - 1];
    }
    mint ans = 0;
    // 最大値をたす
    rep(i, 0, N - K + 1) {
        if (N - i - 2 < 0) break;
        ans += mint(A[i]) * cumsum[N - i - 2];
    }
    reverse(all(A));
    // 最小値を引く
    rep(i, 0, N - K + 1) {
        if (N - i - 2 < 0) break;
        ans -= mint(A[i]) * cumsum[N - i - 2];
    }
    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 4224