import java.io.*;
import java.util.*;

public class C
{
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    // Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        int k = Integer.parseInt(tok.nextToken());
        Song[] songs = new Song[n];
        for (int i = 0; i < n; i++)
        {
            tok = new StringTokenizer(in.readLine());
            songs[i] = new Song(Integer.parseInt(tok.nextToken()), Integer.parseInt(tok.nextToken()), i);
        }
        Arrays.sort(songs);
        PriorityQueue<Song> pq = new PriorityQueue<>((a, b) -> {
            if (a.length == b.length)
            {
                return Integer.compare(a.index, b.index);
            }
            return Long.compare(a.length, b.length);
        });
        pq.add(songs[n-1]);
        long curr = songs[n-1].length;
        long min = songs[n-1].beauty;
        long best = curr * min;
        // out.println(pq);
        // out.println(curr * min);
        for (int i = n-2; i >= 0; i--)
        {
            Song s = songs[i];
            if (pq.size() < k)
            {
                pq.add(s);
                curr += s.length;
                min = s.beauty;
            }
            else
            {
                pq.add(s);
                curr += s.length;
                Song rem = pq.remove();
                curr -= rem.length;
                min = s.beauty;
            }
            best = Math.max(curr * min, best);
            // out.println(pq);
            // out.println(curr * min);
            // out.println("   " + best);
        }
        out.println(best);

        out.flush();
        in.close();
    }

    public static void main(String[] args) throws IOException
    {
        new C().go();
    }

    private class Song implements Comparable<Song>
    {
        long length, beauty;
        int index;

        public Song(long l, long b, int i)
        {
            length = l;
            beauty = b;
            index = i;
        }

        public int compareTo(Song other)
        {
            if (beauty == other.beauty)
            {
                if (length == other.length)
                {
                    return Integer.compare(index, other.index);
                }
                return Long.compare(length, other.length);
            }
            return Long.compare(beauty, other.beauty);
        }

        public String toString()
        {
            return String.format("(%d,%d)", length, beauty);
        }
    }
}
