// #define DEBUGGING
#include <bits/stdc++.h>
#define endl '\n'
#define ALL(V) (V).begin(), (V).end()
#define ALLR(V) (V).rbegin(), (V).rend()
using ll = std::int64_t;
using ull = std::uint64_t;
using PLL = std::pair<ll, ll>;
using TLL = std::tuple<ll, ll, ll>;
template <typename T> using V = std::vector<T>;
template <typename T> using VV = V<V<T>>;
template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return std::min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return std::max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return std::max(low, std::min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { return t = clamp(t, low, high); }
namespace init__ { struct InitIO { InitIO() { std::cin.tie(nullptr); std::ios_base::sync_with_stdio(false); std::cout << std::fixed << std::setprecision(30); } } init_io; }
#define mv_rec make_v(init, tail...)
template <typename T> T make_v(T init) { return init; }
template <typename T, typename... Tail> auto make_v(T init, size_t s, Tail... tail) { return V<decltype(mv_rec)>(s, mv_rec); }
#undef mv_rec
using namespace std;

#ifdef DEBUGGING
#include "../../debug/debug.cpp"
#else
#define DEBUG(...) 0
#define DEBUG_SEPARATOR_LINE 0
#endif

const ll mod = 1e9 + 7;

template <ll MOD>
class Combination {
private:
    ll N;
    V<ll> factv, ifactv;

public:
    Combination<MOD>(ll N) : N(N), factv(N + 1, 1), ifactv(N + 1) {
        for(ll i = 1; i <= N; i++) factv[i] = factv[i - 1] * i % MOD;
        ifactv.back() = inv(factv.back());
        for(ll i = N - 1; 0 <= i; i--) ifactv[i] = (i + 1) * ifactv[i + 1] % MOD;
    }

    ll fact(ll n) { return factv[n]; }

    ll ifact(ll n) { return ifactv[n]; }

    ll pow(ll a, ll b) { return b ? (b & 1 ? a : 1) * pow(a * a % MOD, b / 2) % MOD : 1; }

    ll inv(ll a) { return pow(a, MOD - 2); }

    ll comb(ll n, ll k) { return factv[n] * ifactv[n - k] % MOD * ifactv[k] % MOD; }
};

Combination<mod> C(1e6);

// 0-indexed
// [l, r)
template <typename T>
struct Bit {
    V<T> data;
    T id_ele;

    Bit(ll size, T id_ele) : id_ele(id_ele) {
        ll bsize = 1;
        while(bsize < size) bsize *= 2;
        data = V<T>(bsize + 1, id_ele);
    }

    T sum(ll pos) {
        T ret = id_ele;
        for(; 0 < pos; pos -= pos & -pos) ret += data[pos];
        return ret;
    }

    T sum(ll l, ll r) { return sum(r) - sum(l); }

    void add(ll pos, T delta) {
        for(++pos; pos < data.size(); pos += pos & -pos) data[pos] += delta;
    }
};

int main() {
    ll N, K;
    cin >> N >> K;
    V<ll> A(N);
    for (ll &e : A) cin >> e;
    sort(ALL(A));
    map<ll, ll> id;
    for (ll e : A) {
        if (id.find(e) != id.end()) continue;
        ll tmp = id.size();
        id[e] = tmp;
    }

    Bit<ll> minb(N + 10, 0), maxb(N + 10, 0);
    ll ans = 0;
    for (ll e : A) {
        ll ok = minb.sum(0, id[e] + 1);
        DEBUG(ok);
        minb.add(id[e], 1);
        if (ok < K - 1) continue;
        ll c = C.comb(ok, K - 1);
        ans += e * c % mod;
    }
    reverse(ALL(A));
    for (ll e : A) {
        ll ok = maxb.sum(id[e], N + 10);
        maxb.add(id[e], 1);
        if (ok < K - 1) continue;
        ll c = C.comb(ok, K - 1);
        ans += mod - e * c % mod;
    }

    cout << ans % mod << endl;
    return 0;
}


Measured memory_bytes: 24448