#include <bits/stdc++.h>

using namespace std;
typedef long long int ll;
typedef pair <ll,ll> P;
#define REP(i, n) for(ll i = 0LL; i < (ll)(n); i++)
#define REPR(i, n) for(ll i = n-1; i > 0LL; i--)
#define FOR(i, n, m) for(ll i = n; i < (ll)(m); i++)
#define PRINT(x) cout << x << endl
#define ALL(v) v.begin(), v.end()
#define INF (ll)1e18
#define PI 3.14159265358979
#define MOD 1000000007

ll gcd(ll a, ll b) { return b ? gcd(b,a%b) : a;}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

template<int mod>
struct ModInt{
	int x;
	ModInt():x(0){}
	ModInt(long long y):x(y>=0?y%mod:(mod-(-y)%mod)%mod){}
	ModInt &operator+=(const ModInt &p){
		if((x+=p.x)>=mod)x-=mod;
		return *this;
	}
	ModInt &operator-=(const ModInt &p){
		if((x+=mod-p.x)>=mod)x-=mod;
		return *this;
	}
	ModInt &operator*=(const ModInt &p){
		x=(int)(1LL*x*p.x%mod);
		return *this;
	}
	ModInt &operator/=(const ModInt &p){
		*this*=p.inverse();
		return *this;
	}
	ModInt &operator^=(long long p){
		ModInt res = 1;
		for (; p; p >>= 1) {
			if (p & 1) res *= *this;
			*this *= *this;
		}
		return *this = res;
	}
	ModInt operator-()const{return ModInt(-x);}
	ModInt operator+(const ModInt &p)const{return ModInt(*this)+=p;}
	ModInt operator-(const ModInt &p)const{return ModInt(*this)-=p;}
	ModInt operator*(const ModInt &p)const{return ModInt(*this)*=p;}
	ModInt operator/(const ModInt &p)const{return ModInt(*this)/=p;}
	ModInt operator^(long long p)const{return ModInt(*this)^=p;}
	bool operator==(const ModInt &p)const{return x==p.x;}
	bool operator!=(const ModInt &p)const{return x!=p.x;}
	explicit operator int() const { return x; }						   // added by QCFium
	ModInt operator=(const int p) {x = p; return ModInt(*this);} // added by QCFium
	ModInt inverse()const{
		int a=x,b=mod,u=1,v=0,t;
		while(b>0){
			t=a/b;
			a-=t*b;
			std::swap(a,b);
			u-=t*v;
			std::swap(u,v);
		}
		return ModInt(u);
	}
	friend std::ostream &operator<<(std::ostream &os,const ModInt<mod> &p){
		return os<<p.x;
	}
	friend std::istream &operator>>(std::istream &is,ModInt<mod> &a){
		long long x;
		is>>x;
		a=ModInt<mod>(x);
		return (is);
	}
};
typedef ModInt<MOD> mint;


struct MComb {
    std::vector<mint> fact;
    std::vector<mint> inversed;
    MComb(ll n) { // O(n+log(mod))
        fact = std::vector<mint>(n+1,1);
        for (ll i = 1; i <= n; i++) fact[i] = fact[i-1]*mint(i);
        inversed = std::vector<mint>(n+1);
        inversed[n] = fact[n] ^ (MOD-2);
        for (ll i = n - 1; i >= 0; i--) inversed[i]=inversed[i+1]*mint(i+1);
    }
    mint ncr(ll n, ll r) {
        return (fact[n] * inversed[r] * inversed[n-r]);
    }
    mint npr(ll n, ll r) {
        return (fact[n] * inversed[n-r]);
    }
    mint nhr(ll n, ll r) {
        assert(n+r-1 < (ll)fact.size());
        return ncr(n+r-1, r);
    }
};


int main()
{
    ll N, K;
    cin >> N >> K;

    vector<ll> A(N);
    REP(i, N) cin >> A[i];
    mint ans = 0;
	sort(ALL(A));
    MComb mc(N);
    FOR(i, K-1, N) {
		// PRINT(A[i]);
		// PRINT(i << " C " << K-1);
        ans += (mint)A[i] * mc.ncr(i, K-1);
    }
	// PRINT(ans);
    REP(i, N-K+1) {
		// PRINT(A[i]);
		// PRINT(N-i-1<< " C " << K-1);
        ans -= (mint)A[i] * mc.ncr(N-i-1, K-1);
    }
    PRINT(ans);
    return 0;
}


Measured memory_bytes: 1792