#input of int(split by space)
def get_i()
  return gets.chomp.split(" ").map(&:to_i)
end
#input of float(split by space)
def get_f()
  return gets.chomp.split(" ").map(&:to_f)
end
#input of string(split by space)
def get()
  return gets.chomp.split(" ")
end
#input of string(split per one character)
def get_nsp()
  return gets.chomp.split("")
end
#yes or no decision
def yn_judge(bool,y="Yes",n="No")
  return bool ? y : n 
end
#create of array
def array(size1,init=nil,size2=-1)
  if size2==-1
      return Array.new(size1){init}
  else
      return Array.new(size2){Array.new(size1){init}}
  end
end

def max(a,b)
    return a>b ? a : b
end

def min(a,b)
    return a>b ? b : a
end

INF=Float::INFINITY

N,K=get_i
A=get_i.sort
MOD=10**9+7
$fac=array(N+1)
$finv=array(N+1)
$inv=array(N+1)
def comb_init()
  $fac[0]=$fac[1]=1
  $finv[0]=$finv[1]=1
  $inv[1]=1
  2.upto(N) do|i|
      $fac[i]=$fac[i-1]*i%MOD
      $inv[i]=MOD-$inv[MOD%i]*(MOD/i)%MOD
      $finv[i]=$finv[i-1]*$inv[i]%MOD
  end
end

def comb(n,k)
    return 0 if n<k
    return 0 if n<0 or k<0
    return $fac[n]*($finv[k]*$finv[n-k]%MOD)%MOD
end
comb_init
ans=0
A.each_with_index do|a,i|
  ans+=(comb(i,K-1)-comb(N-i-1,K-1))*a
  ans%=MOD
end
puts ans