import java.util.*;
import java.lang.*;
import java.math.*;
import java.io.*;

/* abhi2601 */

public class Q1 implements Runnable{

    final static long mod = (long)1e9 + 7;

    class Pair{
        int t,b;
        Pair(int t,int b){
            this.t=t;
            this.b=b;
        }
    }
    class Comp implements Comparator<Pair>{
        public int compare(Pair p1, Pair p2){
            if(p1.b<p2.b) return 1;
            if(p1.b>p2.b) return -1;
            return 0;
        }
    }
    class Comp1 implements Comparator<Pair>{
        public int compare(Pair p1, Pair p2){
            if(p1.t<p2.t) return -1;
            if(p1.t>p2.t) return 1;
            return 0;
        }
    }
    public void run() {
        InputReader sc = new InputReader(System.in);
        PrintWriter w = new PrintWriter(System.out);
        int n=sc.nextInt();
        int k=sc.nextInt();
        ArrayList<Pair>al=new ArrayList<>();
        for(int i=0;i<n;i++){
            al.add(new Pair(sc.nextInt(),sc.nextInt()));
        }
        Collections.sort(al,new Comp());
        PriorityQueue<Pair>pq=new PriorityQueue<>(new Comp1());
        int i=0;
        long ans=0L,sum=0L;
        while(i<n){
            int min=al.get(i).b;
            while(i<n && al.get(i).b==min){
                if(pq.size()==k){
                    if(pq.peek().t<al.get(i).t){
                        sum-=pq.poll().t;
                        sum+=al.get(i).t;
                        pq.add(new Pair(al.get(i).t,al.get(i).b));
                    }
                }
                else{
                    sum+=al.get(i).t;
                    pq.add(new Pair(al.get(i).t,al.get(i).b));
                }
                i++;
            }
            ans=Math.max(ans,sum*(long)min);
        }
        w.println(ans);
        w.close();
    }
    static class InputReader {

        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream)
        {
            this.stream = stream;
        }

        public int read()
        {
            if (numChars==-1)
                throw new InputMismatchException();

            if (curChar >= numChars)
            {
                curChar = 0;
                try
                {
                    numChars = stream.read(buf);
                }
                catch (IOException e)
                {
                    throw new InputMismatchException();
                }

                if(numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public String nextLine()
        {
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            String str = "";
            try
            {
                str = br.readLine();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return str;
        }
        public int nextInt()
        {
            int c = read();

            while(isSpaceChar(c))
                c = read();

            int sgn = 1;

            if (c == '-')
            {
                sgn = -1;
                c = read();
            }

            int res = 0;
            do
            {
                if(c<'0'||c>'9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));

            return res * sgn;
        }

        public long nextLong()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-')
            {
                sgn = -1;
                c = read();
            }
            long res = 0;

            do
            {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));
            return res * sgn;
        }

        public double nextDouble()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-')
            {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.')
            {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, nextInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.')
            {
                c = read();
                double m = 1;
                while (!isSpaceChar(c))
                {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, nextInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public String readString()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do
            {
                res.appendCodePoint(c);
                c = read();
            }
            while (!isSpaceChar(c));

            return res.toString();
        }

        public boolean isSpaceChar(int c)
        {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next()
        {
            return readString();
        }

        public interface SpaceCharFilter
        {
            public boolean isSpaceChar(int ch);
        }
    }

    public static void main(String args[]) throws Exception
    {
        new Thread(null, new Q1(),"q1",1<<26).start();
    }
}