#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define countof(array) (sizeof(array) / sizeof(array[0]))
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = (n)-1; i >= 0; --i)
#define rep2(i,n) for(int i = 1; i <= (n); ++i)
#define rrep2(i,n) for(int i = (n); i > 0; --i)
#define srep(i,s,n) for(int i = s; i < (n); ++i)
#define rsrep(i,s,n) for(int i = (n)-1; i >= s; --i)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define aall(a) (a), (a)+countof(a)//for array sorting
#define raall(a) (a), (a)+countof(a), greater<>()
#define show(x) cout<<#x<<" = "<<(x)<<endl;
#define vfind(v, a) find(all(v), a) != v.end()
#define yn(f) { if (f) puts("YES"); else puts("NO"); }
#define yns(f) { if (f) puts("Yes"); else puts("No"); }
#define show_ary(...) { cout<<#__VA_ARGS__<<" = "; for (const auto& x : (__VA_ARGS__)) { cout<<x<<" "; } cout<<endl; }
#define show_pair(...) cout<<#__VA_ARGS__<<" = "<<endl; for (const auto& x : (__VA_ARGS__)) { cout<<"  "<<x.fi<<" : "<<x.se<<endl; }
#define out_ary(...) { int n = (__VA_ARGS__).size(); rep(i, n) printf("%d%s", (__VA_ARGS__)[i], i != n-1 ? " " : "\n"); }
#define argmax(v) distance((v).begin(), max_element(all(v)))
#define argmin(v) distance((v).begin(), min_element(all(v)))
#define vmax(v) *max_element(all(v))
#define vmin(v) *min_element(all(v))
typedef long long int ll;
typedef pair<int, int> P;
typedef vector<P> vpair;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<double> vdouble;
typedef vector<string> vstr;
typedef vector<bool> vbool;
typedef vector<vint> vvint;
typedef vector<vll> vvll;
typedef vector<vstr> vvstr;
typedef vector<vbool> vvbool;
const ll LINF = 2000000000000000000ll;
const int INF = 1000000100;
const ll MOD = 1e9+7;


const int MAX = 200000;
ll fac[MAX], finv[MAX], inv[MAX];

void comb_init() {
    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 % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

ll 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] % MOD) % MOD;
}

struct mint {
    ll x;
    mint(ll x=0):x(x%MOD){}
    mint& operator+=(const mint a) {
        if ((x += a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator-=(const mint a) {
        if ((x += MOD-a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator*=(const mint a) {
        (x *= a.x) %= MOD;
        return *this;
    }
    mint operator+(const mint a) const {
        mint res(*this);
        return res+=a;
    }
    mint operator-(const mint a) const {
        mint res(*this);
        return res-=a;
    }
    mint operator*(const mint a) const {
        mint res(*this);
        return res*=a;
    }
};

int main() {
    int n, k;
    cin >> n >> k;
    vll a(n);
    rep(i, n) cin >> a[i];
    sort(all(a));

    comb_init();

    mint ans = 0;
    vll num(n, 0);
    rep(i, n) {
        num[i] += comb(i, k-1);
        num[i] -= comb(n-i-1, k-1);
    }

    rep(i, n) ans += a[i] * num[i];

    cout << ans.x << endl;
    return 0;
}

Measured memory_bytes: 6528