from heapq import *
def abc144c():
  n,k=map(int,input().split())
  a=sorted(list(map(int,input().split())))
  f=sorted(list(map(int,input().split())))[::-1]
  q=[] 
  for i,j in zip(a,f):
    heappush(q,(-i*j,j))
  kk=min(k,10**12)
  for i in range(k):
    tmp,key=heappop(q)
    heappush(q,(tmp+key,key))
  ans,key=heappop(q)
  print(max(-ans,0))
abc144c()
