#define _USE_MATH_DEFINES
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <clocale>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

#define IOS ios::sync_with_stdio(false); cin.tie(0);
#define FOR(i, s, n) for(int i = (s), i##_len=(n); i < i##_len; ++i)
#define FORS(i, s, n) for(int i = (s), i##_len=(n); i <= i##_len; ++i)
#define VFOR(i, s, n) for(int i = (s); i < (n); ++i)
#define VFORS(i, s, n) for(int i = (s); i <= (n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define REPS(i, n) FORS(i, 0, n)
#define VREP(i, n) VFOR(i, 0, n)
#define VREPS(i, n) VFORS(i, 0, n)
#define RFOR(i, s, n) for(int i = (s), i##_len=(n); i >= i##_len; --i)
#define RFORS(i, s, n) for(int i = (s), i##_len=(n); i > i##_len; --i)
#define RREP(i, n) RFOR(i, n, 0)
#define RREPS(i, n) RFORS(i, n, 0)
#define ALL(v) (v).begin(), (v).end()
#define SORT(v) sort(ALL(v))
#define RSORT(v) sort(ALL(v), greater<decltype(v[0])>())
#define SZ(x) ((int)(x).size())
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(ALL(v)), v.end())

using ll = long long;
using ui = unsigned int;
using ull = unsigned long long;
using Pi_i = pair<int, int>;
using VB = vector<bool>;
using VC = vector<char>;
using VD = vector<double>;
using VI = vector<int>;
using VLL = vector<ll>;
using VS = vector<string>;
using VSH = vector<short>;
using VULL = vector<ull>;

const int MOD = 1000000007;
const int INF = 1000000000;
const int NIL = -1;
const double EPS = 1E-10;

template<class T, class S>
bool chmax(T &a, const S &b){
    if(a < b){
        a = b; return true;
    }
    return false;
}
template<class T, class S>
bool chmin(T &a, const S &b){
    if(b < a){
        a = b; return true;
    }
    return false;
}


VLL fact_inv, fact;

ll power(ll a, ll b, ll m){
    ll ret = 1;
    while(b){
        if(b & 1) ret = (ret * a) % m;
        a = (a * a) % m;
        b /= 2;
    }
    return ret;
}

ll power(ll a, ll b) {
    return power(a, b, MOD);
}

ll inv(ll a, ll m){ // GCD(a, m) = 1
    return power(a, m-2);
}

ll inv(ll a){
    return inv(a, MOD);
}

ll comb(ll a, ll b, ll m){
    if(a < b) return 0;
    if(a < 0 || b < 0) return 0;
    ll ret = fact[a];
    ret = ret * fact_inv[b] % m;
    ret = ret * fact_inv[a-b] % m;
    return ret;
}

ll comb(ll a, ll b){
    return comb(a, b, MOD);
}

void precom(int n, ll m){
    fact.resize(n);
    fact_inv.resize(n);
    fact[0] = fact[1] = 1;
    fact_inv[0] = fact_inv[1] = 1;
    for(int i = 2; i < n; ++i) fact[i] = fact[i-1] * i % m;
    for(int i = 2; i < n; ++i) fact_inv[i] = inv(fact[i]);
    return;
}

void precom(int n){
    precom(n, MOD);
    return;
}

int main(){
    int N, K; cin >> N >> K;
    VLL A(N);
    precom(N + 3);
    REP(i, N) cin >> A[i];
    RSORT(A);
    ll ans = 0;
    REPS(i, N-K){
        ans = (ans + (A[i] * comb(N-i-1, K-1)) % MOD) % MOD;
        ans = (ans - (A[N-1-i] * comb(N-i-1, K-1)) % MOD) % MOD;
    }
    if(ans < 0) ans += MOD;
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 2560