import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;

public class Main {
    public static void main(String args[])
    {
        FastReader input=new FastReader();
        PrintWriter out=new PrintWriter(System.out);
        int T=1;
        while(T-->0)
        {
            int n=input.nextInt();
            int k=input.nextInt();
            ArrayList<Integer> a[]=new ArrayList[n];
            for(int i=0;i<n;i++)
            {
                a[i]=new ArrayList<>();
                a[i].add(input.nextInt());
                a[i].add(input.nextInt());
            }
            mergeSort(a,0,n-1);
            PriorityQueue<Integer> q=new PriorityQueue<>();
            long sum=0;
            long max=Long.MIN_VALUE;
            for(int i=0;i<n;i++)
            {
                if(i<k)
                {
                    q.add(a[i].get(0));
                    sum+=a[i].get(0);
                    max=Math.max(max,sum*a[i].get(1));
                }
                else
                {
                    int m=q.peek();
                    if(a[i].get(0)>m)
                    {
                        q.poll();
                        sum-=m;
                        sum+=a[i].get(0);
                        max=Math.max(max,sum*a[i].get(1));
                        q.add(a[i].get(0));
                    }
                }
            }
            out.println(max);
        }
        out.close();
    }
    public static void mergeSort(ArrayList<Integer> a[],int p,int r)
    {
        if(p<r)
        {
            int q=(p+r)/2;
            mergeSort(a,p,q);
            mergeSort(a,q+1,r);
            merge(a,p,q,r);
        }
    }
    public static void merge(ArrayList<Integer> a[],int p,int q,int r)
    {
        int n1=q-p+2;
        ArrayList<Integer> L[]=new ArrayList[n1];
        int n2=r-q+1;
        ArrayList<Integer> R[]=new ArrayList[n2];
        for(int i=p;i<=q;i++)
        {
            L[i-p]=a[i];
        }
        L[n1-1]=new ArrayList<>();
        L[n1-1].add(0);
        L[n1-1].add(0);
        for(int i=q+1;i<=r;i++)
        {
            R[i-q-1]=a[i];
        }
        R[n2-1]=new ArrayList<>();
        R[n2-1].add(0);
        R[n2-1].add(0);
        int x=0,y=0;
        for(int i=p;i<=r;i++)
        {
            if(L[x].get(1)>R[y].get(1))
            {
                a[i]=L[x];
                x++;
            }
            else if(L[x].get(1)<R[y].get(1))
            {
                a[i]=R[y];
                y++;
            }
            else
            {
                if(L[x].get(0)>=R[y].get(0))
                {
                    a[i]=L[x];
                    x++;
                }
                else
                {
                    a[i]=R[y];
                    y++;
                }
            }
        }
    }
    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new
                    InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}