/*
 *created by Kraken on 12-04-2020 at 14:54
 */
//package com.kraken.cf;

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

public class C1140 {
    static class Pair {
        long x, y;
        Pair (long x, long y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public String toString() {
            return "Pair{" +
                "x=" + x +
                ", y=" + y +
                '}';
        }
    }

    public static void main(String[] args) {
        FastReader sc = new FastReader();
        int n = sc.nextInt(), k = sc.nextInt();
        Pair[] pairs = new Pair[n];
        for (int i = 0; i < n; i++) {
            pairs[i] = new Pair(sc.nextLong(), sc.nextLong());
        }
        Arrays.sort(pairs, (o1, o2) -> {
            if (o1.y == o2.y) return (int) (o2.x - o1.x);
            else return (int) (o2.y - o1.y);
        });
//        System.out.println(Arrays.toString(pairs));
        long res = Long.MIN_VALUE;
        long duration = 0;
        PriorityQueue<Long> queue = new PriorityQueue<>();
        for (int i = 0; i < k; i++) {
            duration += pairs[i].x;
            queue.add(pairs[i].x);
            res = Math.max(res, duration * pairs[i].y);
        }
        for (int i = k; i < n; i++) {
            duration += pairs[i].x;
            while (queue.size() > k - 1) duration -= queue.poll();
            queue.add(pairs[i].x);
            res = Math.max(res, duration * pairs[i].y);
        }
        System.out.println(res);
    }

    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;
        }
    }
}
