#include <algorithm>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define rep(i, n) for(int(i) = 0; (i) < (n); (i)++)
#define FOR(i, m, n) for(int(i) = (m); (i) < (n); (i)++)
#define All(v) (v).begin(), (v).end()
#define pb push_back
#define MP(a, b) make_pair((a), (b))
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int INF = 1 << 30;
const ll LINF = 1LL << 60;
const int MOD = 1e9 + 7;

template <uint_fast64_t MOD> class ModInt {
    using u64 = uint_fast64_t;

  public:
    u64 a;

    ModInt(const u64 x = 0) : a(x % MOD) {}
    constexpr u64 &value() { return a; }
    constexpr ModInt operator-() { return a ? MOD - a : 0; }
    constexpr ModInt operator+(const ModInt rhs) {
        return ModInt(*this) += rhs;
    }
    constexpr ModInt operator-(const ModInt rhs) {
        return ModInt(*this) -= rhs;
    }
    constexpr ModInt operator*(const ModInt rhs) {
        return ModInt(*this) *= rhs;
    }
    constexpr ModInt operator/(const ModInt rhs) {
        return ModInt(*this) /= rhs;
    }
    constexpr ModInt &operator+=(const ModInt rhs) {
        a += rhs.a;
        if(a >= MOD) {
            a -= MOD;
        }
        return *this;
    }
    constexpr ModInt &operator-=(const ModInt rhs) {
        if(a < rhs.a) {
            a += MOD;
        }
        a -= rhs.a;
        return *this;
    }
    constexpr ModInt &operator*=(const ModInt rhs) {
        a = a * rhs.a % MOD;
        return *this;
    }
    constexpr ModInt &operator/=(const ModInt rhs) {
        *this *= rhs.inv();
        return *this;
    }

    constexpr bool operator==(const ModInt rhs) { return this->a == rhs.a; }
    constexpr bool operator!=(const ModInt rhs) { return this->a != rhs.a; }
    friend constexpr ostream &operator<<(ostream &os, const ModInt<MOD> &x) {
        return os << x.a;
    }
    friend constexpr istream &operator>>(istream &is, ModInt<MOD> &x) {
        return is >> x.a;
    }

    constexpr ModInt inv() {
        u64 x = a, y = MOD, u = 1, v = 0, t = 0;
        while(y > 0) {
            t = x / y;
            a -= t * y;
            swap(x, y);
            u -= t * v;
            swap(u, v);
        }
        return ModInt(u);
    }

    constexpr ModInt pow(int e) {
        u64 x = 1, p = a;
        while(e > 0) {
            if(e % 2 == 0) {
                p = (p * p) % MOD;
                e /= 2;
            } else {
                x = (x * p) % MOD;
                e--;
            }
        }
        return ModInt(x);
    }
};

//二項係数（nCk mod.p;1<=k<=n<=1e7,pは素数）for ModInt
template <class T> struct BiCoefficient {
    const int MOD = 1000000007;
    vector<T> fac, finv, inv;
    BiCoefficient(const int MAX) : fac(MAX), finv(MAX), inv(MAX) {
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[1] = 1;
        for(int i = 2; i < MAX; i++) {
            fac[i] = fac[i - 1] * i;
            inv[i] = -inv[MOD % i] * (MOD / i);
            finv[i] = finv[i - 1] * inv[i];
        }
    }
    T comb(int n, int k) {
        if(n < k)
            return 0;
        if(n < 0 || k < 0)
            return 0;
        return fac[n] * finv[k] * finv[n - k];
    }
};

using mint = ModInt<MOD>;

int main() {
    int N, K;
    cin >> N >> K;
    vector<ll> A(N);
    rep(i, N) cin >> A[i];
    sort(All(A));
    BiCoefficient<mint> Bicoef(100010);
    mint res = 0;
    for(int i = N - 1; i >= K - 1; i--) {
        if(A[i] < 0) {
            res -= mint(-A[i]) * Bicoef.comb(i, K - 1);

        } else {
            res += mint(A[i]) * Bicoef.comb(i, K - 1);
        }
    }
    for(int i = 0; N - 1 - i >= K - 1; i++) {
        if(A[i] < 0) {
            res += mint(-A[i]) * Bicoef.comb(N - 1 - i, K - 1);
        } else {
            res -= mint(A[i]) * Bicoef.comb(N - 1 - i, K - 1);
        }
    }
    cout << res << endl;
    return 0;
}

Measured memory_bytes: 3328