#include<bits/stdc++.h>
using namespace std;using ll=long long;using vi=vector<int>;using vvi=vector<vi>;using vl=vector<ll>;using vvl=vector<vl>;using P=pair<int,int>;using PL=pair<ll,ll>;using vp=vector<P>;using vpl=vector<PL>;
#define pqueue priority_queue
template<typename T>constexpr auto inf=numeric_limits<T>::max()/2;constexpr int INF=inf<int>,MOD=1000000007;constexpr ll LINF=inf<ll>;
#define _ol3(_1,_2,_3,name,...)name
#define _rep(i,n)_repi(i,0,n)
#define _repi(i,a,b)for(int i=a,i##_l=(b);i<i##_l;++i)
#define REP(...)_ol3(__VA_ARGS__,_repi,_rep,)(__VA_ARGS__)
#define REPR(i,n)for(int i=n-1;i>=0;--i)
#define REPA(i,v)REP(i,(v).size())
#define all(v)(v).begin(),(v).end()
#define rall(v)(v).rbegin(),(v).rend()
#define bit(n)(1ll<<(n))
#define F first
#define S second
#define endl '\n'
#define cho(n,a,b)cout<<((n)?a:b)<<endl
void YES(int n){cho(n,"YES","NO");}void Yes(int n){cho(n,"Yes","No");}void Poss(int n){cho(n,"Possible","Impossible");}
void _print(ostream&){}template<class T,class...U>void _print(ostream&s,const T&t,const U&...u){s<<t<<(sizeof...(u)?' ':'\n');_print(s,u...);}
template<class...T>void print(const T&...t){_print(cout,t...);}template<class...T>void dprint(const T&...t){_print(cerr,t...);}
#ifndef LOCAL
struct osd{template<class T>osd&operator<<(const T&t){return*this;}};osd cer_;
#define dprint(...)
#define cerr cer_
#endif
#define dbg(x) dprint("@l",__LINE__,':',#x,'=',x)
template<class T>bool chmax(T&a,const T&b){bool x=a<b;x?a=b:b;return x;}template<class T>bool chmin(T&a,const T&b){bool x=a>b;x?a=b:b;return x;}
template<class T,class U>ostream&operator<<(ostream&o,const pair<T,U>&p){return o<<p.F<<' '<<p.S;}
template<class T,class U>istream&operator>>(istream&i,pair<T,U>&p){return i>>p.F>>p.S;}
template<class T>class iterable{static false_type c(string v);template<class U>static auto c(U v)->decltype(all(v),true_type());static false_type c(...);public:const static bool value=decltype(c(declval<T>()))::value;};
template<class T,enable_if_t<iterable<T>::value,int> =0>ostream&operator<<(ostream&o,const T&v){for(auto&&i:v)o<<i<<' ';return o;}
template<class T>istream&operator>>(istream&i,vector<T>&v){for(T&j:v)i>>j;return i;}template<class T>vector<T>&operator<<(vector<T>&v,const T&t){v.push_back(t);return v;}
template<class T,class U>queue<T,U>&operator<<(queue<T,U>&v, const T&t){v.push(t);return v;}template<class T,class U>queue<T,U>&operator>>(queue<T,U>&v, T&t){t=v.front();v.pop();return v;}
template<class T,class U,class V>pqueue<T,U,V>&operator<<(pqueue<T,U,V>&v, const T&t){v.push(t);return v;}template<class T,class U,class V>pqueue<T,U,V>&operator>>(pqueue<T,U,V>&v, T&t){t=v.front();v.pop();return v;}
template<class T,class U>stack<T,U>&operator<<(stack<T,U>&v, const T&t){v.push(t);return v;}template<class T,class U>stack<T,U>&operator>>(stack<T,U>&v, T&t){t=v.front();v.pop();return v;}
template<class T,class U,class V>set<T,U,V>&operator<<(set<T,U,V>&v,const T&t){v.insert(t);return v;}template<class T,class U,class V>multiset<T,U,V>&operator<<(multiset<T,U,V>&v,const T&t){v.insert(t);return v;}
template<class T,enable_if_t<is_integral<T>::value,int> =0>T mod(T a, T b){if(a>0)return a%b;return b-(-a%b);}
template<class T,enable_if_t<is_floating_point<T>::value,int> =0>T mod(T a, T b){if(a>0)return a%b;return b-fmod(-a,b);}
template<class T,class U>pair<T,U>&operator+=(pair<T,U>&a, pair<T,U>&b){a.F+=b.F;a.S+=b.S;return a;}template<class T,class U>pair<T,U>operator+(const pair<T,U>&a, const pair<T,U>&b){T r=a;return r+=b;}
template<class T,class U>pair<T,U>&operator-=(pair<T,U>&a, pair<T,U>&b){a.F-=b.F;a.S-=b.S;return a;}template<class T,class U>pair<T,U>operator-(const pair<T,U>&a, const pair<T,U>&b){T r=a;return r-=b;}
template<class T,class U>pair<T,U>operator-(pair<T,U>&a){return mp(-a.F, -a.S);}
inline ll square(ll a){a%=MOD;return (a*a)%MOD;}
ll lpow(ll a,ll b){
    a%=MOD;
    b%=MOD-1;
    if(b==0)return 1;
    if(b==1)return a;
    if(b%2){
        return (square(lpow(a,b/2)%MOD)*a)%MOD;
    }else{
        return square(lpow(a,b/2)%MOD);
    }
}

vl ft;
ll fact(ll n){
    // ll ans=1;
    // while(n){
    //     ans=(ans*n--)%MOD;
    // }
    // return ans;
    return ft[n];
}

ll factto(ll a,ll b){
    ll ans=1;
    for(ll i=a;i<=b;i++){
        ans=(ans*i)%MOD;
    }
    return ans;
}

ll perm(ll n,ll r){ // nPr
    //return factto(n-r+1,n)%MOD;
    return fact(n)*(lpow(fact(n-r), MOD-2))%MOD;
}

ll comb(ll n,ll r){ // nCr
    if(r<0)return 0;
    if(r>n)return 0;
    return (perm(n,r)*lpow(fact(r),MOD-2))%MOD;
}

ll homo(ll n,ll r){ // nHr
    return comb(n+r-1,r);
}
int main(){
    cin.tie(0);ios::sync_with_stdio(0);
    cout<<setprecision(10);
    int n,k;
    cin>>n>>k;
    ft.resize(n+1);
    ft[0]=1;
    REP(i,1,n+1)ft[i]=ft[i-1]*i%MOD;
    vi a(n);
    cin>>a;
    sort(all(a));
    ll ans=0;
    REP(i,n){
        dprint('-', n-i-1, 'C', k-1, '*', a[i], '=', comb(n-i-1, k-1) * a[i]);
        ans-=comb(n-i-1, k-1) * a[i] % MOD;
        (ans+=MOD)%=MOD;
        dprint('+', i, 'C', k-1, '*', a[i], '=', comb(i, k-1) * a[i]);
        ans+=comb(i, k-1) * a[i] % MOD;
        ans%=MOD;
    }
    cout<<ans<<endl;
}


Measured memory_bytes: 1408