#include <iostream>
#include <algorithm>

using namespace std;

const long MAX = 510000;
const long MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    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;
    }
}

// 二項係数計算
long long COM(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;
}

int main(){
    int N, K;
    cin >> N >> K;

    long A[N];
    for(int i=0; i<N; i++){
        cin >> A[i];
    }

    sort(A, A+N);

    COMinit();

    long ans = 0;
    for(int i=N-1; i>=0; i--){
        ans = ans + A[i]*COM(i, K-1) - A[i]*COM(N-i-1, K-1);
        ans = ans % MOD;
    }

    printf("%ld\n", ans);

    return 0;
}