#include "bits/stdc++.h"
// Begin Header {{{
#define all(x)          (x).begin(), (x).end()
#define rep(i, n)       for (i64 i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, s, t)   for (i64 i = (s), i##_limit = (t); i <= i##_limit; ++i)
#define repr(i, s, t)   for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#ifndef DBG
#define trace(...)
#endif
using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template <class T, class U> inline bool chmax(T &a, const U &b) { return b > a && (a = b, true); }
template <class T, class U> inline bool chmin(T &a, const U &b) { return b < a && (a = b, true); }
constexpr int INF  = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template <class T>
inline vector<T> make_v(const T &initValue, size_t sz) {
    return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto make_v(const T &initValue, size_t sz, Args... args) {
    return vector<decltype(make_v<T>(initValue, args...))>(sz, make_v<T>(initValue, args...));
}

inline void read() {}

template <class Head, class... Tail>
inline void read(Head &head, Tail&... tail) { cin >> head; read(tail...); }

inline void print() { cout << "\n"; }

template <class Head, class... Tail>
inline void print(Head &&head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) cout << ' ';
    print(forward<Tail>(tail)...);
}

template <class T>
inline ostream& operator<< (ostream &out, const vector<T> &vec) {
    static constexpr const char *delim[] = { " ", "" };
    for (const auto &e : vec) out << e << delim[&e == &vec.back()];
    return out;
}
// }}} End Header

constexpr int MOD = int(1e9) + 7;

// ModInt {{{
template<int64_t MOD>
class ModInt {
    int64_t value;
public:
    // constructor
    inline ModInt(int64_t val = 0) noexcept :
        value((val >= MOD) ? (val % MOD) : (val < 0) ? ((val + MOD) % MOD) : val) {}

    // プリミティブ整数型へのキャスト (型の明示:必要)
    template<class Int>
        explicit inline operator Int() const noexcept { return static_cast<Int>(value); }

    inline ModInt inv() const noexcept {
        return ModInt<MOD>::pow(value, MOD-2);
    }

    inline ModInt& operator+=(ModInt x) noexcept {
        if ((value += x.value) >= MOD) value -= MOD;
        return *this;
    }
    inline ModInt& operator-=(ModInt x) noexcept {
        if ((value -= x.value) < 0) value += MOD;
        return *this;
    }
    inline ModInt& operator*=(ModInt x) noexcept {
        value = (value * x.value) % MOD; return *this;
    }
    inline ModInt& operator/=(ModInt x) noexcept {
        value = (value * x.inv().value) % MOD; return *this;
    }

    inline ModInt operator+(ModInt x) const noexcept { return ModInt(*this) += x; }
    inline ModInt operator-(ModInt x) const noexcept { return ModInt(*this) -= x; }
    inline ModInt operator*(ModInt x) const noexcept { return ModInt(*this) *= x; }
    inline ModInt operator/(ModInt x) const noexcept { return ModInt(*this) /= x; }
    inline bool operator==(ModInt x) const noexcept { return value == x.value; }
    inline bool operator!=(ModInt x) const noexcept { return !(this->operator==(x)); }

    friend ostream& operator<<(ostream &os, ModInt x) noexcept { os << x.value; return os; }
    friend istream& operator>>(istream &is, ModInt &x) noexcept { is >> x.value; x.value %= MOD; return is; }

    static constexpr inline ModInt pow(int64_t n, int64_t p) noexcept {
        int64_t ret = 1;
        for(; p > 0; p >>= 1) {
            if (p & 1) ret = (ret * n) % MOD;
            n = (n * n) % MOD;
        }
        return ret;
    }
};
// }}}

using modint = ModInt<MOD>;

// Factorial, InvFact {{{
template<size_t N, int MOD>
struct Factorial {
    uint_fast64_t f[N+1];

    inline Factorial() : f{1} {
        for (int i = 1; i <= N; ++i) f[i] = (f[i-1] * i) % MOD;
    }

    constexpr inline ModInt<MOD> operator[] (size_t i) const { return f[i]; }
};

template<size_t N, int MOD>
struct InvFact {
    uint_fast64_t inv[N+1];
    uint_fast64_t f[N+1];

    inline InvFact() : inv{0, 1}, f{1, 1} {
        for (int i = 2; i <= N; ++i) {
            inv[i] = (MOD - MOD/i) * inv[MOD % i] % MOD;
            f[i] = f[i-1] * inv[i] % MOD;
        }
    }

    constexpr inline ModInt<MOD> operator[] (size_t i) const { return f[i]; }
};
// }}}

Factorial<200010, MOD> fact;
InvFact<200010, MOD> invFact;

ModInt<MOD> nCr(int n, int r) { return (r < 0 || n < r) ? 0 : (fact[n] * (invFact[r] * invFact[n-r])); }
ModInt<MOD> nPr(int n, int r) { return (r < 0 || n < r) ? 0 : (fact[n] * invFact[n-r]); }
ModInt<MOD> nHr(int n, int r) { return nCr(n+r-1, r); }

signed main() {
    ios_base::sync_with_stdio(false); cin.tie(nullptr);

    int N, K;
    read(N, K);

    vector<i64> a(N);
    rep(i, N) read(a[i]);
    sort(all(a));

    modint ans = 0;
    rep(i, N) {
        const int left = i;
        const int right = N - (i + 1);
        trace(left, right);
        ans += modint(a[i]) * nCr(left, K-1);
        ans -= modint(a[i]) * nCr(right, K-1);
    }

    print(ans);

    return 0;
}


Measured memory_bytes: 5760