import java.io.*;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.StringTokenizer;

public class Solution {
    static final int INF = (int) 1e9;
    static final int mod = (int) (1e9 + 7);
    static final short UNCALC = -1;

    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        int n = sc.nextInt();
        int k = sc.nextInt();
        Song[] songs = new Song[n];
        for (int i = 0; i < n; i++)
            songs[i] = new Song(sc.nextInt(), sc.nextInt());
        Arrays.sort(songs);
        long ans = 0;
        PriorityQueue<Integer> pq = new PriorityQueue<>();
        long sum = 0;
        for (int i = n - 1; i >= 0; i--) {
            if (pq.size() == k) {
                int min = pq.remove();
                sum -= min;
                sum += songs[i].len;
                ans = Math.max(ans, sum * songs[i].b);
                sum -= songs[i].len;
                int put = Math.max(min, songs[i].len);
                pq.add(put);
                sum += put;
            } else {
                sum += songs[i].len;
                pq.add(songs[i].len);
                ans = Math.max(ans, sum * songs[i].b);
            }
        }
        out.println(ans);
        out.flush();
        out.close();
    }


    static class Song implements Comparable<Song> {
        int len, b;

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

        @Override
        public int compareTo(Song o) {
            return b == o.b ? o.len - len : b - o.b;
        }
    }

    static class Scanner {
        StringTokenizer st;
        BufferedReader br;

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

        public Scanner(String file) throws Exception {
            br = new BufferedReader(new FileReader(file));
        }

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

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

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

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

        public char nextChar() throws IOException {
            return next().charAt(0);
        }

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

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

        public void waitForInput() throws InterruptedException {
            Thread.sleep(3000);
        }

        public int[] nextIntArray(int n) throws IOException {
            int[] a = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }

        public Integer[] nextIntegerArray(int n) throws IOException {
            Integer[] a = new Integer[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }

        public double[] nextDoubleArray(int n) throws IOException {
            double[] ans = new double[n];
            for (int i = 0; i < n; i++)
                ans[i] = nextDouble();
            return ans;
        }

        public short nextShort() throws IOException {
            return Short.parseShort(next());
        }

    }
}