import os
import sys
from io import BytesIO, IOBase
import heapq

BUFSIZE = 8192

class FastIO(IOBase):
    newlines = 0
 
    def __init__(self, file):
        import os
        self.os = os
        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 = self.os.read(self._fd, max(self.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 = self.os.read(self._fd, max(self.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:
            self.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")

# "". join(strings)  
    
def ri():
    return int(input())
 
def rl():
    return list(map(int, input().split()))
    

n, k = rl()
beauty_length=[]
for i in range(n):
    t, b = rl()
    beauty_length.append((b, t))

beauty_length.sort()

k_longuest = []
length = 0
ans = 0
for i in range(n):
    b, t = beauty_length.pop()

    if len(k_longuest) < k:
        heapq.heappush(k_longuest,t)
        length += t
        ans = max(ans, length * b)
    else:
        t_shortest = k_longuest[0]
        if t_shortest < t:
            heapq.heappop(k_longuest)
            heapq.heappush(k_longuest,t)
            length += t - t_shortest
            ans = max(ans, length * b)
print(ans)












