#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
#define repr(i,n) for(int i=n-1;i>=0;i--)
#define MAX(a,b) a=a>b?a:b
#define MIN(a,b) a=a<b?a:b
#define REP(i,x,n) for(int i=x;i<n;i++)
#define REPR(i,x,n) for(int i=n-1;i>=x;i--)
#define pb  push_back
#define ALL(obj) (obj).begin(), (obj).end()
#define ALLr(obj) (obj).rbegin(), (obj).rend()
#define endl "\n"
#define F first
#define S second
#define EN cout<<endl;
#define FAST ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
typedef vector<ll> VLL;
typedef vector<VLL> VVLL;
typedef vector<P> VP;
typedef priority_queue<ll> PQ;
const ll mod=1000000007;
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yes(T b){if(b)out("yes");else out("no");}
template<class T> void Yes(T b){if(b)out("Yes");else out("No");}
template<class T> void YES(T b){if(b)out("YES");else out("NO");}
template<class T> void no(T b){if(b)out("no");else out("yes");}
template<class T> void No(T b){if(b)out("No");else out("Yes");}
template<class T> void NO(T b){if(b)out("NO");else out("YES");}
template<class T> int sint(T s){int n;sscanf(s.c_str(),"%d",&n);return n;}
//combsort
template<typename T> void cSort(vector<T>& v){size_t h = (v.size()*10)/13;bool is_sorted = false;while(!is_sorted){if(h==1)is_sorted = true;for(size_t i=0; i<v.size()-h; ++i){if(v[i]>v[i+h]){swap(v[i],v[i+h]);if(is_sorted)is_sorted = false;}}if(h>1) h = (h*10)/13;if(h==0) h = 1;}}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){ll res=1;a%=mod;while(b){if(b&1)res=res*a%mod;a=a*a%mod;b>>=1;}return res;}
//約数列挙
vector<long long> divisor(long long n) {vector<long long> ret;for (long long i = 1; i * i <= n; i++) {if (n % i == 0) {ret.push_back(i);if (i * i != n) ret.push_back(n / i);}}sort(ret.begin(), ret.end()); return ret;}
template <int MOD>
struct ModInt {int val;ModInt(ll v = 0) : val(v % MOD) {if (val < 0) val += MOD;};ModInt operator+() const { return ModInt(val); }ModInt operator-() const { return ModInt(MOD - val); }ModInt inv() const { return this->pow(MOD - 2); }ModInt operator+(const ModInt& x) const { return ModInt(*this) += x; }ModInt operator-(const ModInt& x) const { return ModInt(*this) -= x; }ModInt operator*(const ModInt& x) const { return ModInt(*this) *= x; }ModInt operator/(const ModInt& x) const { return ModInt(*this) /= x; }ModInt pow(ll n) const {auto x = ModInt(1);auto b = *this;while (n > 0) {if (n & 1) x *= b;n >>= 1;b *= b;}return x;}ModInt& operator+=(const ModInt& x) {if ((val += x.val) >= MOD) val -= MOD;return *this;}ModInt& operator-=(const ModInt& x) {if ((val -= x.val) < 0) val += MOD;return *this;}ModInt& operator*=(const ModInt& x) {val = ll(val) * x.val % MOD;return *this;}ModInt& operator/=(const ModInt& x) { return *this *= x.inv(); }bool operator==(const ModInt& b) const { return val == b.val; }bool operator!=(const ModInt& b) const { return val != b.val; }friend istream& operator>>(istream& is, ModInt& x) noexcept { return is >> x.val; }friend ostream& operator<<(ostream& os, const ModInt& x) noexcept { return os << x.val; }};
constexpr int MOD = 1e9 + 7;
using mint = ModInt<MOD>;



class Combination
{
    vector<long long> fact;
    vector<long long> factinv;
    long long MOD = 1e9 + 7;
    long long modpow(long long a, long long n)
    {
        if (n == 0)
            return 1;
        if (n % 2 == 0)
        {
            long long t = modpow(a, n / 2);
            return t * t % MOD;
        }
        return a * modpow(a, n - 1) % MOD;
    }

    long long modinv(long long a)
    {
        // mod is prime
        return modpow(a, MOD - 2);
    }

public:
    Combination(int n) : fact(n), factinv(n)
    {
        fact[0] = 1;
        for (int i = 1; i < n; i++)
        {
            fact[i] = fact[i - 1] * i % MOD;
        }
        factinv[n - 1] = modinv(fact[n - 1]);
        for (int i = n - 2; i >= 0; i--)
        {
            factinv[i] = factinv[i + 1] * (i + 1) % MOD;
        }
    }

    long long ncr(long long n, long long r)
    {
        if (r < 0 || r > n)
            return 0;
        return fact[n] * factinv[r] % MOD * factinv[n - r] % MOD;
    }

    long long npr(long long n, long long r)
    {
        if (r < 0 || r > n)
            return 0;
        return fact[n] * factinv[n - r] % MOD;
    }

    long long nhr(long long n, long long r)
    {
        if (n == 0 && r == 0)
            return 1;
        return ncr(n + r - 1, r);
    }
};

int main() {
    ll n,k;
    
    cin>>n>>k;
    Combination comb(n+10);
    VLL a;
    rep(i,n){
        ll b;
        cin>>b;
        a.pb(b);
    }
    ll de[101010]={};//
    sort(ALL(a));
    mint ans=0;
    mint co[101010];
    REP(i,k,n+1){
        co[i-k]=comb.ncr(i-1,k-1);
    }
    rep(i,n-k+1){
        ans+=co[i]*a[i+k-1];
        ans-=co[n-k-i]*a[i];
    }
    cout<<ans<<endl;
    
    
}


Measured memory_bytes: 5992