#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (n); ++i)
#define per(i, n) for(int i = (n)-1; i >= 0; --i)
#define eee(a) (cerr << (#a) << ": " << (a) << endl);
#define ppp(a) (cout << (a) << endl); 
#define cop(a) for(auto x:(a)){cout << x << endl;}

using ll = long long;

constexpr int dx[4] = {1, 0, -1, 0};
constexpr int dy[4] = {0, 1, 0, -1};

constexpr char sp = ' ';
constexpr ll MOD = 1e9+7;
constexpr ll Mod = 998244353;
constexpr int INF = 1e9;
constexpr ll LINF = 1e18;
constexpr double phi = (1+sqrt(5))/2;
constexpr double pi = 3.14159265358979323846264338327950288;
inline ll gcd(ll a, ll b){return b?gcd(b,a%b):a;}
inline ll lcm(ll a, ll b){return a/gcd(a,b)*b;}
inline ll inverse(ll n, ll m=MOD){ll a=n,b=m,x=1,y=0;while(b){ll t=a/b;a-=t*b;swap(a,b);x-=t*y;swap(x,y);}return (a%m+m)%m;}
inline void Yes(){puts("Yes");}
inline void No(){puts("No");}
inline void Judge(bool b){b?Yes():No();}
inline void YES(){puts("YES");}
inline void NO(){puts("NO");}
inline void JUDGE(bool b){b?YES():NO();}
inline ll powmod(ll b, ll e, ll m=MOD){ll r=1;while(e>0){if(e&1)r=(r%m)*(b%m)%m;b=(b%m)*(b%m)%m;e>>=1;}return r;}
inline double distance(ll x1, ll y1, ll x2, ll y2){return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}
inline double d2r(double d){return d*pi/180;}
inline double r2d(double r){return r*180/pi;}

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename T1, typename T2> ostream& operator<<(ostream& s, const pair<T1, T2>& p) {return s << "(" << p.first << ", " << p.second << ")";}
template<typename T> ostream& operator<<(ostream& s, const vector<T>& v) {
    s<<'[';for(size_t i=0;i<v.size();++i){s << v[i];if(i < v.size()-1) s << ", ";}s<<']';return s;
}
struct __{ __(){ cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(15); } };

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;
    }
};

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> coe;

int main(){

    coe.init(100100);
    ll n, k;
    scanf("%lld%lld", &n, &k);
    vector<ll> A(n);
    vector<mint> a(n);
    rep(i, n) scanf("%lld", &A[i]);
    sort(A.begin(), A.end());
    rep(i, n) a[i] = mint(A[i]);

    mint f = 0;
    for(int i = 1; n-i >= k-1; ++i){
        f += a[n-i]*mint(coe.com(n-i, k-1));
        f -= a[i-1]*mint(coe.com(n-i, k-1));
    }
    ppp(f);

    return 0;
}



Measured memory_bytes: 4224