//package com.company;
// Always comment out package when submitting.

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

public class Main {
    public static class Task {

        public class Song {
            int t, b, idx;

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

        public void solve(Scanner sc, PrintWriter pw) throws IOException {
            int n = sc.nextInt();
            int k = sc.nextInt();
            int[][] songs = new int[n][2];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < 2; j++) {
                    songs[i][j] = sc.nextInt();
                }
            }
            Arrays.sort(songs, new Comparator<int[]>() {
                @Override
                public int compare(int[] o1, int[] o2) {
                    return Integer.compare(o1[1], o2[1]);
                }
            });
            PriorityQueue<Song> pq = new PriorityQueue<>(new Comparator<Song>() {
                @Override
                public int compare(Song o1, Song o2) {
                    if (o1.t == o2.t) {
                        return o2.b - o1.b;
                    }
                    return o2.t - o1.t;
                }
            });
            for (int i = 0; i < n; i++) {
                pq.add(new Song(songs[i][0], songs[i][1], i));
            }
            boolean[] validIdx = new boolean[n];
            boolean[] isIdx = new boolean[n];
            long tot = 0;
            for (int i = 0; i < k; i++) {
                Song s = pq.poll();
                tot += s.t;
                isIdx[s.idx] = true;

            }
            long best = 0;
            for (int i = 0; i < n; i++) {
                best = Math.max(best, tot * songs[i][1]);
                validIdx[i] = true;
                if (isIdx[i]) {
                    isIdx[i] = false;
                    tot -= songs[i][0];
                    while (pq.size() > 0 && validIdx[pq.peek().idx]) pq.poll();
                    if (pq.size() > 0) {
                        Song s = pq.poll();
                        isIdx[s.idx] = true;
                        tot += s.t;
                    }
                }
            }
            pw.println(best);
        }
    }

    // template, actual code is in class Task.
    static long TIME_START, TIME_END;

    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(System.in);
//        Scanner sc = new Scanner(new FileInputStream("Test.in"));
        PrintWriter pw = new PrintWriter(new BufferedOutputStream(System.out));
//        PrintWriter pw = new PrintWriter(new FileOutputStream("Test.out"));
        Runtime runtime = Runtime.getRuntime();
        long usedMemoryBefore = runtime.totalMemory() - runtime.freeMemory();
        TIME_START = System.currentTimeMillis();
        Task t = new Task();
        t.solve(sc, pw);
        TIME_END = System.currentTimeMillis();
        long usedMemoryAfter = runtime.totalMemory() - runtime.freeMemory();
        pw.close();
//        System.err.println("Memory increased: " + (usedMemoryAfter - usedMemoryBefore) / 1000000);
//        System.err.println("Time used: " + (TIME_END - TIME_START) + ".");
    }

    // Faster IO with BufferedReader wrapped with Scanner
    static class Scanner {
        StringTokenizer st;
        BufferedReader br;

        public Scanner(InputStream s) {
            br = new BufferedReader(new InputStreamReader(s));
        }

        public Scanner(FileReader s) throws FileNotFoundException {
            br = new BufferedReader(s);
        }

        public String next() throws IOException {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(br.readLine());
            return st.nextToken();
        }

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

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

        public String nextLine() throws IOException {
            return br.readLine();
        }

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

        public boolean ready() throws IOException {
            return br.ready();
        }
    }
}