#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
using namespace std;
using ll = long long;
using vec = vector<ll>;
using vect = vector<double>;
using Graph = vector<vector<ll>>;
#define loop(i, n) for (ll i = 0; i < n; i++)
#define Loop(i, m, n) for (ll i = m; i < n; i++)
#define pool(i, n) for (ll i = n; i >= 0; i--)
#define Pool(i, m, n) for (ll i = n; i >= m; i--)
#define mod 1000000007ll
#define flagcount __builtin_popcount
#define flag(x) (1 << x)
#define flagadd(bit, x) bit |= flag(x)
#define flagpop(bit, x) bit &= ~flag(x)
#define flagon(bit, i) bit &flag(i)
#define flagoff(bit, i) !(bit & (1 << i))
#define all(v) v.begin(), v.end()
#define low2way(v, x) lower_bound(all(v), x)
#define high2way(v, x) upper_bound(all(v), x)
#define idx_lower(v, x) (distance(v.begin(), low2way(v, x)))  //配列vでx未満の要素数を返す
#define idx_upper(v, x) (distance(v.begin(), high2way(v, x))) //配列vでx以下の要素数を返す
#define idx_lower2(v, x) (v.size() - idx_lower(v, x))         //配列vでx以上の要素数を返す
#define idx_upper2(v, x) (v.size() - idx_upper(v, x))         //配列vでxより大きい要素の数を返す
#define putout(a) cout << a << endl
#define Gput(a, b) G[a].push_back(b)
#define Sum(v) accumulate(all(v), 0ll)
#define gcd(x, y) __gcd(x, y)
ll ctoi(char c)
{
    if (c >= '0' && c <= '9')
    {
        return c - '0';
    }
    return 0;
}
template <typename T>
T lcm(T x, T y)
{
    T z = gcd(x, y);
    return x * y / z;
}
template <typename T>
bool primejudge(T n)
{
    if (n < 2)
        return false;
    else if (n == 2)
        return true;
    else if (n % 2 == 0)
        return false;
    double sqrtn = sqrt(n);
    for (T i = 3; i < sqrtn + 1; i++)
    {
        if (n % i == 0)
        {
            return false;
        }
        i++;
    }
    return true;
}
//場合によって使い分ける
//const ll dx[4]={1,0,-1,0};
//const ll dy[4]={0,1,0,-1};
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
//2次元配列の宣言
//vector<vector<ll>> field(h, vector<ll>(w));
long long modpow(long long a, long long n)
{
    long long res = 1;
    while (n > 0)
    {
        if (n & 1)
            res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
long long modinv(long long a)
{
    return modpow(a, mod - 2);
}
vec fact(200001);    //fact[i]=(i!)
vec factinv(200001); //factinv[i]=(i!)^-1
void COMinit()
{
    long long now = 1;
    fact[0] = 1;
    factinv[0] = modinv(0);
    Loop(i, 1, 200001)
    {
        now *= i;
        now %= mod;
        fact[i] = now;
        factinv[i] = modinv(now);
    }
}
long long COM(long long n, long long r)
{
    if (n < r)
        return 0;
    if (n < 0 || r < 0)
        return 0;
    if (n == r)
        return 1;
    if (r == 0)
        return 1;
    long long ans = fact[n];
    ans *= factinv[r];
    ans %= mod;
    ans *= factinv[n - r];
    ans %= mod;
    return ans;
}
int main()
{
    cout << fixed << setprecision(30);
    ll N, K;
    cin >> N >> K;
    vec A(N);
    loop(i, N) cin >> A[i];
    sort(all(A));
    ll ans = 0;
    COMinit();
    //maxXを足していく
    loop(i, N - K + 1)
    {
        ans += (A[(K - 1) + i] * COM(K - 1 + i, K - 1)) % mod;
        ans %= mod;
    }
    //minXを引いていく
    loop(i, N - K + 1)
    {
        ans -= (A[N - K - i] * COM(K - 1 + i, K - 1)) % mod;
        ans += mod;
        ans %= mod;
    }
    putout(ans);
    return 0;
}


Measured memory_bytes: 7088