#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long LL;
typedef vector<ll>VI;
typedef vector<bool>VB;
typedef pair<ll,ll>P;
typedef pair<double,double>P_D;
#define VV(T) vector<vector<T>>
#define PI 3.1415926535897932384626433832795
#define rep(i, n) for (ll i = 0; i < (ll)n; i++)
#define reprev(i, n) for (ll i = (ll)n - 1; i >= 0; i--)
#define bitrep(bit,A) for(int bit=A; ;bit=(bit-1)&A)
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) (ll)((a).size())
#define c_max(a, b) (((ll)a)>((ll)b)?(a):(b))
#define c_min(a,b) (((ll)a)<((ll)b)?(a):(b))
#define chmax(x,a) x=max(x,a)
#define chmin(x,a) x=min(x,a)
#define vmax(v) *max_element(ALL(v))
#define vmin(v) *min_element(ALL(v))
#define SORT(c) stable_sort(ALL(c))
#define $(x) {cout<<#x<<" = " <<(x)<<endl;}
#define fi first
#define se second
#define MAX 100100//5
#define MAX2 200100
#define MAX_6 1001001//6
#define MAX_7 10010010//7
#define SENTINEL 2000000000//9
#define NIL -1
const int MOD = 1000000007;
#define INF 1<<30
#define INFTY 1LL<<61
#define MAX_INT INT_MAX
#define Endl '\n'
inline ll GCD(ll a,ll b){return b?GCD(b,a%b):a;}
inline ll lcm(ll a,ll b){return a*b/GCD(a,b);}
using Edge=pair<int,ll>;
using Graph=vector<vector<int> >;//ll or Edge
using Edge=pair<int,ll>;
const int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
// sort_reverse...greater<ll>()
// g++ -o a a.cpp -Wall -lm -std=c++17
template<int MOD> struct Fp {
    long long val;
    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if (val < 0) val += MOD;
    }
    constexpr int getmod() { return MOD; }
    constexpr Fp operator - () const noexcept {
        return val ? MOD - val : 0;
    }
    constexpr Fp operator + (const Fp& r) const noexcept { return Fp(*this) += r; }
    constexpr Fp operator - (const Fp& r) const noexcept { return Fp(*this) -= r; }
    constexpr Fp operator * (const Fp& r) const noexcept { return Fp(*this) *= r; }
    constexpr Fp operator / (const Fp& r) const noexcept { return Fp(*this) /= r; }
    constexpr Fp& operator += (const Fp& r) noexcept {
        val += r.val;
        if (val >= MOD) val -= MOD;
        return *this;
    }
    constexpr Fp& operator -= (const Fp& r) noexcept {
        val -= r.val;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr Fp& operator *= (const Fp& r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }
    constexpr Fp& operator /= (const Fp& r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while (b) {
            long long t = a / b;
            a -= t * b; swap(a, b);
            u -= t * v; swap(u, v);
        }
        val = val * u % MOD;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr bool operator == (const Fp& r) const noexcept {
        return this->val == r.val;
    }
    constexpr bool operator != (const Fp& r) const noexcept {
        return this->val != r.val;
    }
    friend constexpr ostream& operator << (ostream &os, const Fp<MOD>& x) noexcept {
        return os << x.val;
    }
    friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
        if (n == 0) return 1;
        auto t = modpow(a, n / 2);
        t = t * t;
        if (n & 1) t = t * a;
        return t;
    }
};

// 二項係数ライブラリ
template<class T> struct BiCoef {
    vector<T> fact_, inv_, finv_;
    constexpr BiCoef() {}
    constexpr BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) {
        init(n);
    }
    constexpr void init(int n) noexcept {
        fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1);
        int MOD = fact_[0].getmod();
        for(int i = 2; i < n; i++){
            fact_[i] = fact_[i-1] * i;
            inv_[i] = -inv_[MOD%i] * (MOD/i);
            finv_[i] = finv_[i-1] * inv_[i];
        }
    }
    constexpr T com(int n, int k) const noexcept {
        if (n < k || n < 0 || k < 0) return 0;
        return fact_[n] * finv_[k] * finv_[n-k];
    }
    constexpr T fact(int n) const noexcept {
        if (n < 0) return 0;
        return fact_[n];
    }
    constexpr T inv(int n) const noexcept {
        if (n < 0) return 0;
        return inv_[n];
    }
    constexpr T finv(int n) const noexcept {
        if (n < 0) return 0;
        return finv_[n];
    }
};
using mint=Fp<MOD>;
BiCoef<mint>bc;



int main(){
    bc.init(510000);
    ll n,k;
    cin>>n>>k;
    VI A(n);
    rep(i,n)cin>>A[i];
    sort(ALL(A));
    mint res=0;
    rep(i,n-1){
        mint tmp=A[i+1]-A[i];
        mint fac=bc.com(n,k)-bc.com(i+1,k)-bc.com(n-i-1,k);
        res=(res+fac*tmp);
    }
    cout<<res<<endl;
}

Measured memory_bytes: 15892