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//package EducationRound62;

import java.util.Arrays;
import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Scanner;

/**
 *
 * @author Hemant Dhanuka
 */
public class C {
    public static void main(String[] args) {
        Scanner s=new Scanner(System.in);
        
        int n=s.nextInt();
        int k=s.nextInt();
        
        Node nodes[]=new  Node[n];
        for(int i=0;i<n;i++){
            Node node=new Node(s.nextInt(), s.nextInt());
            nodes[i]=node;
        }
        
        
        Arrays.sort(nodes,new Comparator<Node>() {
            @Override
            public int compare(Node o1, Node o2) {
                
                if(o1.b!=o2.b){
                    return o1.b-o2.b;
                }else{
                    return o2.t-o1.t;
                }
            }
        });
        
         int counter=1;
         long max=0;
         long sum=0;
         PriorityQueue<Integer> pq=new PriorityQueue<>();
         for(int i=n-1;i>=0;i--){
             if(counter<=k){
                 pq.add(nodes[i].t);
                 sum=sum+nodes[i].t;
                 counter++;
             }
             else{
                 sum=sum-pq.poll();
                 pq.add(nodes[i].t);
                 sum=sum+nodes[i].t;
             }
             
             
             max=Math.max(max,sum*nodes[i].b);
             
             
             
             
             
         }
         
         System.out.println(max);

    }
    
    
    
    
    static class Node{
       int t;
       int b;

        public Node(int t, int b) {
            this.t = t;
            this.b = b;
        }
       
       
    }
}
