#include<bits/stdc++.h>
using namespace std;

#define MOD 1000000007
#define N 100010

typedef double lf;
typedef long long ll;
#define int ll
typedef long double ld;
typedef unsigned long long ull;
typedef set<ll> sll;
typedef vector<ll> vll;
typedef vector<pair<ll,ll > >vpll;
typedef vector<vll > vvll;
#define endl "\n"
#define sd(a) scanf("%d",&a)
#define sstr(a) scanf("%s",a)
#define sl(a) scanf("%lld",&a)
#define debug(a) printf("check%d\n",a)
#define clr(a) memset(a,0,sizeof(a))
#define rtn return 
#define gc getchar
#define pb push_back
#define ff first
#define ss second
#define mp(x,y) make_pair(x,y)
#define all(a) a.begin(),a.end()
#define allr(a) a.rbegin(),a.rend()
#define lp(i,b) for(ll i = ll(0); i<ll(b) ; i++) 
#define lpit(it,a) for(__typeof(a.begin()) it = a.begin(); it != a.end(); ++it)
#define mid(s,e) (s+(e-s)/2)
#define min3(a,b,c) min(a,min(b,c))
#define max3(a,b,c) max(a,max(b,c))
#define fast ios_base::sync_with_stdio(NULL),cin.tie(0),cout.tie(0);
ll ip(){
  ll x = 0; bool isNeg = false; char c;
  c = gc();
  if(c == '-')  isNeg = true , c = gc();
  for( ; c >= '0'  && c <= '9' ; c = gc()); x = (x << 1) + (x << 3) + c - '0';
  if(isNeg) x *= -1;  return x;
}
//////////////////////////////////////////// CONTAINERS FOR FUNCTIONS MENTIONED IN TEMPLATE ///////////////////////////////////
#define MAX 100005
//bool isPrime[MAX];
//int spf[MAX];
//////////////////////////////////////////// FUNCTIONS THAT NEED COMMENTED OUT CONTAINERS ////////////////////////////////////


//void prime_sieve(){ for(ll i=2 ; i*i<=MAX ; i++){ if(!isPrime[i]){ for(ll j=i*i ; j<=MAX ; j+=i)  isPrime[j] = 1;} }  }
//void spf_sieve(){ for(ll i=2 ; i<MAX ; i+=2)  spf[i] = 2;
//  for(ll i=3 ; i<MAX ; i+=2){ if(!isPrime[i]){ spf[i] = i; for(ll j=i ; (j*i)<MAX ; j+=2){ if(!isPrime[i*j])  isPrime[i*j] = true , spf[i*j] = i; } } }
//}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
inline ll ncr(ll n,ll r){ ll ans = 1 ; if(r > n-r) r = n-r; lp(i,r+1){ans*=(n-i) ; ans/=(i+1); } rtn ans; }
inline ll gcd(ll a,ll b){if(!a) rtn b; rtn gcd(b%a,a); }
inline ll fme(ll x,ll n){ll ans=1;x%=MOD;while(n>0){if(n&1){ ans*=x;ans%=MOD;} x*=x;x%=MOD;n>>=1;}rtn ans%MOD;}
inline bool isPalin(string &s){ int len = s.size()-1;lp(i,(len/2)+2){if(!(s[i]==s[len-i]))  rtn false;} rtn true;}
inline ll lcm(ll a,ll b){rtn (a*b)/gcd(a,b); }
inline ll fmm(ll a,ll b) {ll r=0;a%=MOD;b%=MOD;while(b>0){if(b&1){r+=a;r%=MOD;}a+=a;a%=MOD;b>>=1;}rtn r%MOD;}
inline ll sfme(ll a,ll b) {ll r=1;a%=MOD;while(b>0){if(b&1)r=fmm(r,a);a=fmm(a,a);b>>=1;}rtn r%MOD;}
inline int in(){ int NR=0; register char c=gc(); while( c < 48 || c > 57 ){c=gc();} 
  while(c>47 && c< 58){ NR = (NR << 3) + (NR << 1) + (c - 48); c=gc();} return NR;
}

ll dx[] = {0,0,1,-1}, dy[] = {1,-1,0,0};
//--------------------------------TEMPLATE ENDS HERE--------------------------------------------------//

#define NMAX 100005
#define LOGNMAX 20
int f[NMAX];

int power(int a,int b){
	if(b==0){
		return 1;
	}
	int s=power(a,b/2);
	s*=s;
	s%=MOD;
	if(b&1){
		return (s*a)%MOD;
	}
	return s;
}

int inv(int a){
	return power(a,MOD-2);
}

int nCr(int n,int r){
	if(n<0 || r<0){
		return 0;
	}
	int ans=0;
	ans=f[n];
	ans*=inv(f[r]);
	ans%=MOD;
	ans*=inv(f[n-r]);
	ans%=MOD;
	return ans;
}



signed main(){
	fast;	
	f[0]=1;
	for(int i=1;i<NMAX;i++){
		f[i]=(f[i-1]*i)%MOD;
	}
	int n,k;
	cin>>n>>k;
	vector<int> a(n);
	for(int i=0;i<n;i++){
		cin>>a[i];
	}

	sort(a.begin(),a.end());

	int ans=0;
	int s=0;
	for(int i=k-1;i<n;i++){
		// (k-1)C(k-1),kC(k-1),(k+1)C(k-1),...,(k+x)C(k-1)
		s+=(nCr(i,k-1)*a[i])%MOD;
		s%=MOD;
	}
	for(int i=n-k;i>=0;i--){
		ans+=(nCr(n-i-1,k-1)*a[i])%MOD;
		ans%=MOD;
	}
	ans=(s-ans)%MOD;
	ans=(ans%MOD+MOD)%MOD;
	cout<<ans<<endl;
}

Measured memory_bytes: 1792