#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define rep(i, n) for(int i = 0; i < n; i++)
#define REP(i, a, b) for(int i = a; i < b; i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define fi first
#define se second
#define debug(x) cerr <<__LINE__<< ": " <<#x<< " = " << x << endl
#define debug_vec(v) cerr<<__LINE__<<": "<<#v<<" = ";rep(i,v.size())cerr<<" "<<v[i];cerr<<endl
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF = (1<<30) - 1;
const ll LINF = (1LL<<60) - 1;

template <unsigned int mod>
struct ModInt {
    ll x;
    ModInt() : x(0) {}
    ModInt(ll y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
    ModInt &operator+=(const ModInt &p) { if((x += p.x) >= mod) x -= mod; return *this; }
    ModInt &operator-=(const ModInt &p) { if((x += mod - p.x) >= mod) x -= mod; return *this; }
    ModInt &operator*=(const ModInt &p) { x = (int) (1LL * x * p.x % mod); return *this; }
    ModInt &operator/=(const ModInt &p) { *this *= p.inverse(); return *this; }
    ModInt operator-() const { return ModInt(-x); }
    ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
    ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
    ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
    ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
    bool operator==(const ModInt &p) const { return x == p.x; }
    bool operator!=(const ModInt &p) const { return x != p.x; }
    ModInt pow(int64_t n) const {
        ModInt ret(1), mul(x);
        while(n > 0) {
            if(n & 1) ret *= mul;
            mul *= mul;
            n >>= 1;
        }
        return ret;
    }
    ModInt inverse() const { return pow(mod - 2); }
    friend ostream &operator<<(ostream &os, const ModInt &p) {
        return os << p.x;
    }
};

const int mod = 1000000007;
using mint = ModInt<mod>;

template<class T>
struct Combination {
    int max_n;
    std::vector<T> f, invf;

    explicit Combination(int n)
            : max_n(n), f(n + 1), invf(n + 1) {
        f[0] = 1;
        for (int i = 1; i <= n; ++i) {
            f[i] = f[i - 1] * i;
        }

        invf[max_n] = f[max_n].inverse();
        for (int i = max_n - 1; i >= 0; --i) {
            invf[i] = invf[i + 1] * (i + 1);
        }
    }

    T fact(int n) const { return f[n]; }

    T invfact(int n) const { return invf[n]; }

    T perm(int a, int b) const {
        return a < b || b < 0 ? T(0) : f[a] * invf[a - b];
    }

    T comb(int a, int b) const {
        return a < b || b < 0 ? T(0) : f[a] * invf[a - b] * invf[b];
    }
};

Combination<mint> C(2020202);

void solve() {
    int n, k;
    cin >> n >> k;
    vector<int> A(n);
    rep(i, n) cin >> A[i];
    sort(all(A));
    mint ans = 0;
    REP(i, k - 1, n) {
        ans += C.comb(i, k - 1) * A[i];
    }
    REP(i, 0, n - k + 1) {
        ans -= C.comb(n - i - 1, k - 1) * A[i];
    }
    cout << ans << endl;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}


Measured memory_bytes: 35108