from heapq import *
n,k=list(map(int,input().split()))
a=[]
for i in range(0,n):
	b,p=list(map(int,input().split()))
	a.append([p,b])

a.sort(reverse=True)

r=0
s=0
h=[]
c=1
for p,b in a:
    s+=b
    heappush(h,b)
    if len(h)>k:
	    s-=heappop(h)
    r=max(r,s*p)
print(r)