#------------------------------warmup----------------------------
import os
import sys
from io import BytesIO, IOBase
 
BUFSIZE = 8192
 
 
class FastIO(IOBase):
    newlines = 0
 
    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None
 
    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()
 
    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()
 
    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)
 
 
class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")
 
 
sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")
 
#-------------------game starts now----------------------------------------------------
import heapq
def sort_list(list1, list2): 
  
    zipped_pairs = zip(list2, list1) 
  
    z = [x for _, x in sorted(zipped_pairs)] 
      
    return z 
n,k=map(int,input().split())
a=[0]*n
b=[0]*n
for i in range(n):
    a[i],b[i]=map(int,input().split())
a=sort_list(a,b)
a.reverse()
b.sort(reverse=True)
s=[]
sum=0
ans=0
for i in range(n):
    if i==k:
        heapq.heapify(s)
    if i<k:
        s.append(a[i])
        sum+=a[i]
        ans=max(sum*b[i],ans)
    else:
        e=heapq.heappop(s)
        sum-=e
        sum+=a[i]
        ans=max(sum*b[i],ans)
        if a[i]>e:
            heapq.heappush(s,a[i])
        else:
            heapq.heappush(s,e)
            sum+=e-a[i]
print(ans)
        
        
        
        