#include <bits/stdc++.h>
using namespace std;
#define ll                      long long
#define pb                      push_back
#define rep(i,a,b,c)            for(ll i=(a);i<=(b);i+=(c))
#define repb(i,a,b,c)           for(ll i=(a);i>=(b);i-=(c))
#define MOD                     1000000007
#define inf                     3e18
#define ld                      long double
#define mp                      make_pair
#define vpll                    vector<pair<ll,ll> >
#define vll                     vector<ll>
#define vld                     vector<ld>
#define vvll                    vector<vector<ll>>
#define vvld                    vector<vector<ld>>
#define pll                     pair<ll,ll>
#define vvpll                   vector<vector<pair<ll,ll>>>
#define pqll                    priority_queue<ll>
#define mll                     map<ll,ll>
#define mlc                     map<ll,char>
#define um                      unordered_map
#define umll                    um<ll,ll>
#define umlc                    um<ll,char>
#define all(x)                  x.begin(),x.end()
#define fi                      first
#define se                      second
#define ln                      cout<<"\n";
#define endl                    "\n"
#define test                    ll T;cin>>T;while(T--)
#define isvowel(a)              (a == 'a' || a == 'e' || a == 'i' || a == 'o' || a == 'u')
#define fastIO                  ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define show(w,size)            for(ll i=0;i<size;i++) cout<<w[i]<<" ";
#define prll(a)                 cout<<a<<"\n";
#define mset(dp,no)             memset(dp,no,sizeof(dp))
#define input(a,n)              rep(i,0,n-1,1) cin>>a[i];
#define countbit                __builtin_popcount                                           //Number of setbits in decimal notation
#define lb(v,val)               lower_bound(all(v),val)-v.begin()
#define ub(v,val)               upper_bound(all(v),val)-v.begin()
/*#define db(...)                 __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) { cerr << name << " : " << arg1 <<'\n'; }
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
    const char* comma = strchr(names + 1, ',');
    cout.write(names, comma - names) << " : " << arg1 << " | "; __f(comma + 1, args...);
}*/
ll pows(ll b,ll e){if(e==0)return 1;else if(e%2==0){ll a=pow(b,e/2);return a*a;}else {ll a=pow(b,e/2);return b*a*a;}}
ll powm(ll x,ll y,ll m){x=x%m;ll res=1;while(y){if(y&1)res=res*x;res%=m;y=y>>1;x=x*x;x%=m;}return res;}
//ll modInverse(ll a, ll m=MOD){if(m == 1)return 0;ll m0=m,y=0,x=1;while(a>1){ll q=a/m,t=m;m=a%m,a=t;t=y;y=x-q*y;x=t;}if(x<0) x+=m0;return x;}
ll modInverse(ll n,ll p) 
{ 
    return powm(n, p-2, p); 
} 
ll fact[100005];
ll calc()
{
	fact[0]=1;
	for(ll i=1;i<100005;i++)
	{
		fact[i]=fact[i-1]*i;
		fact[i]%=MOD;
	}
}
ll nCr(ll n,ll r,ll p=MOD)
{ 
// Base case 
if(n==0||n<r)
{
return 0;
}
if(n==r||r==0) 
{
return 1; 
}
return (fact[n]*modInverse(fact[r],p)%p*modInverse(fact[n-r],p)%p)%p; 
}
int main()
{
fastIO
calc();
ll n,m,i,j,k,x,y,z,e,f,p,q,g,l,r,w,h,count1=0,prod=1,a,b,c,d,index,x1,x2,diff,ans=0,sum=0,sum1=0,sum2=0,flag=0,flag1=0,flag2=0;
string s,s1,s2;
cin>>n>>k;
ll ar[n];
for(i=0;i<n;i++)
{
	cin>>ar[i];
}
if(k==1)
{
	cout<<"0";
	return 0;
}
sort(ar,ar+n);
k--;
for(i=0;i<n;i++)
{
p=nCr(n-i-1,k);
q=nCr(i,k);
sum+=(q-p)*ar[i];
sum=(sum+MOD)%MOD;
}
cout<<sum;
}

Measured memory_bytes: 1792