#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> Pii;
typedef pair<ll,ll> Pll;
#define rep(i,n) for (ll i=0;i<(n);++i)
#define rep2(i,a,b) for (ll i=(a);i<(b);++i)
#define debug(x) cout << #x << '=' << x << endl
#define all(v) (v).begin(),(v).end()
const ll MOD=1e9+7;
//const ll MOD=998244353;
const ll INF=1e9;
const ll IINF=1e18;
const double EPS=1e-8;
const double pi=acos(-1);

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

template<uint_fast64_t Modulus> class modint{
    using u64=uint_fast64_t;
    public:
    u64 a;
    constexpr modint(const u64 x=0) noexcept:a(x%Modulus){}
    constexpr u64 &value() noexcept{return a;}
    constexpr const u64 &value() const noexcept{return a;}
    constexpr modint &operator+=(const modint &rhs) noexcept{
        a+=rhs.a;
        if (a>=Modulus) a-=Modulus;
        return *this;
    }
    constexpr modint operator+(const modint &rhs) const noexcept{
        return modint(*this)+=rhs;
    }
    constexpr modint &operator++() noexcept{
        return ++a,*this;
    }
    constexpr modint operator++(int) noexcept{
        modint t=*this; return ++a,t;
    }
    constexpr modint &operator-=(const modint &rhs) noexcept{
        if (a<rhs.a) a+=Modulus;
        a-=rhs.a;
        return *this;
    }
    constexpr modint operator-(const modint &rhs) const noexcept{
        return modint(*this)-=rhs;
    }
    constexpr modint &operator--() noexcept{
        return --a,*this;
    }
    constexpr modint operator--(int) noexcept{
        modint t=*this; return --a,t;
    }
    constexpr modint &operator*=(const modint &rhs) noexcept{
        a=a*rhs.a%Modulus;
        return *this;
    }
    constexpr modint operator*(const modint &rhs) const noexcept{
        return modint(*this)*=rhs;
    }
    constexpr modint &operator/=(const modint &rhs) noexcept{
        u64 exp=Modulus-2;
        while(exp){
            if (exp&1) *this*=rhs;
            rhs*=rhs; exp>>=1;
        }
    }
    constexpr modint operator/(const modint &rhs) const noexcept{
        return modint(*this)/=rhs;
    }
    constexpr modint operator-() const noexcept{
        return modint(Modulus-a);
    }
    constexpr bool operator==(const modint &rhs) const noexcept{
        return a==rhs.a;
    }
    constexpr bool operator!=(const modint &rhs) const noexcept{
        return a!=rhs.a;
    }
    constexpr bool operator!() const noexcept{return !a;}
    friend ostream &operator<<(ostream &s,const modint &rhs) noexcept {
        return s << rhs.a;
    }
    friend istream &operator>>(istream &s,modint &rhs) noexcept {
        u64 a; rhs=modint{(s >> a,a)}; return s;
    }
};

using mint=modint<MOD>;

const int MAX=5e5+10;
vector<mint> fac(MAX),finv(MAX),inv(MAX);
void COMinit(){
    fac[0]=fac[1]=1;
    finv[0]=finv[1]=1;
    inv[1]=1;
    rep2(i,2,MAX){
        fac[i]=fac[i-1]*i;
        inv[i]=-inv[MOD%i]*(MOD/i);
        finv[i]=finv[i-1]*inv[i];
    }
}
mint COM(int n,int k){
    if (n<k||n<0||k<0) return 0;
    return fac[n]*finv[k]*finv[n-k];
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll N,K; cin >> N >> K;
    vector<ll> A(N);
    rep(i,N) cin >> A[i];
    sort(all(A));
    COMinit();
    mint ans=0;
    rep(i,N){
        ans+=mint(MOD+A[i])*COM(i,K-1);
        ans-=mint(MOD+A[i])*COM(N-i-1,K-1);
    }
    cout << ans << endl;
}

Measured memory_bytes: 12800