using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	static void Main(){
		Sc sc=new Sc();
		var s=sc.Ia;
		var a=sc.Ia;
		int n=s[0],m=s[1];
		Array.Sort(a);
		Cb r=new Cb(n+1);
		int u=n-m+1;
		var z=new Mint[n-m+1];
		for(int i = 0;i<n-m+1;i++) {
			z[i]=r.C1(m-1+i,i);
		}
		var ra=new long[n+1];
		for(int i = 1;i<=n;i++) {
			ra[i]=a[i-1]+ra[i-1];
		}
		int g=0;
		Mint rr=0,ans=0;
		for(int i = n-1;i>=0;i--) {
			if(u-(n-i)<0){
				g=i+1;
				break;
			}
			ans-=a[n-i-1]*z[u-(n-i)];
		}
		for(int i = g,j=1;i<n;i++,j++) {
			rr+=ra[j];
			ans+=z[u-(n-i)]*a[i];
		}
		Console.WriteLine(ans);
	}
}
public struct Mint{
	const int mod=(int)1e9+7;
	private long d;
	public Mint(long d){this.d=d;}
	public static implicit operator Mint(long d){return new Mint(d%mod);}
	public static explicit operator long(Mint d){return d.d;}
	public override string ToString(){return d.ToString();}
	public static Mint operator+(Mint a,long b){a.d=(a.d+b)%mod;return a;}
	public static Mint operator+(Mint a,Mint b){a.d=(a.d+b.d)%mod;return a;}
	public static Mint operator-(Mint a,long b){a.d=(mod+a.d-b)%mod;return a;}
	public static Mint operator-(Mint a,Mint b){a.d=(mod+a.d-b.d)%mod;return a;}
	public static Mint operator*(Mint a,long b){a.d=(a.d*b)%mod;return a;}
	public static Mint operator*(Mint a,Mint b){a.d=(a.d*b.d)%mod;return a;}
	public static Mint operator/(Mint a,long b){a.d=(a.d*Mi(b))%mod;return a;}
	public static Mint operator/(Mint a,Mint b){a.d=(a.d*Mi(b.d))%mod;return a;}
	private static long Mi(long a){
		a=(a+mod)%mod;
		long b=mod,u=1,v=0;
		while(b>0){
			long t=a/b;
			a-=t*b;a^=b;b^=a;a^=b;
			u-=t*v;u^=v;v^=u;u^=v;
		}
		u%=mod;
		if(u<0){u+=mod;}
		return u%mod;
	}
}
public class Cb{
	private const int mod=(int)1e9+7;
	public long[] r;
	public Cb(int n){
		r=new long[n];
		r[0]=1;
		for(int i=1;i<n;i++){r[i]=(r[i-1]*i)%mod;}
	}
	public long C1(long n,long k){return (r[n]*Mi(r[k]*r[n-k]))%mod;}
	public long C2(long n,long k){
		long l=1;
		n%=mod;
		for(int i=1;i<=k;i++){l=(l*(n-i+1))%mod;l=(l*Mi(i))%mod;}
		return l;
	}
	public long P1(long n,long k){return n<k?0:(r[n]*Mi(r[n-k]))%mod;}
	public long Mi(long a){
		a=(a+mod)%mod;
		long b=mod,u=1,v=0;
		while(b>0){long t=a/b;a-=t*b;a^=b;b^=a;a^=b;u-=t*v;u^=v;v^=u;u^=v;}
		u%=mod;
		if(u<0){u+=mod;}
		return u%mod;
	}
}
public class Pe<T>{
	public int o,n,cnt=0;
	private T[] arr,h;
	private bool[] b;
	public List<T[]> li;
	public Pe(int u,int v){o=v-u+1;n=o;Fu1(u);}
	public Pe(int u,int v,int x){o=v-u+1;n=x;Fu1(u);}
	public Pe(T[] a){n=a.Length;Fu2(a);}
	public Pe(T[] a,int x){n=x;Fu2(a);}
	public void Fu1(int u){
		h=new T[o];
		for(int i=0;i<o;i++){h[i]=(T)(object)(i+u);}
		Fu();
	}
	public void Fu2(T[] a){
		o=a.Length;
		h=new T[o];
		Array.Copy(a,h,o);
		Fu();
	}
	public void Fu(){
		b=new bool[o];
		arr=new T[n];
		li=new List<T[]>();
		Rc(0);
	}
	private void Rc(int a){
		for(int i=0;i<o;i++){
			if(!b[i]){
				arr[a]=h[i];
				if(a==n-1){
					li.Add(new T[n]);
					Array.Copy(arr,li[li.Count-1],n);
					cnt++;
				}
				else{b[i]=true;Rc(a+1);b[i]=false;}
			}
		}
	}
}
public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}

Measured memory_bytes: 30940