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

public class Main {
    static PrintWriter pw;
    static Scanner sc;
    static HashMap<Pair, Integer> map;
    static String end;
    static String st;
    static int inf=(int)1e9;
    static long ceildiv(long x, long y){ return (x+y-1)/y;}
    public static void main(String[] args) throws Exception {
        sc = new Scanner(System.in);
        pw = new PrintWriter(System.out);
        int n=sc.nextInt(), k=sc.nextInt();
        Pair[] arr=sc.nextPairArr(n, n);
        Arrays.sort(arr, Collections.reverseOrder());
        long max=0;
        long sum=0;
        int min=inf;
        for(int i=0; i<k; i++){
            sum+=arr[i].x;
            min=Math.min(min, arr[i].y);
            max=Math.max(sum*min, max);
        }
        pw.println(max);
        pw.close();
    }
    static void fill(boolean[][] arr) throws IOException{
        Pair st=sc.nextPair(), end=sc.nextPair();
        for(int i=st.x-1; i<end.x; i++){
            for(int j=st.y-1; j<end.y; j++)
                arr[i][j]=true;
        }
    }
    static void printArr(int[] arr){
        for(int i=0; i<arr.length-1; i++)
            pw.print(arr[i]+" ");
        pw.println(arr[arr.length-1]);
    }
    static void printArr(long[] arr){
        for(int i=0; i<arr.length-1; i++)
            pw.print(arr[i]+" ");
        pw.println(arr[arr.length-1]);
    }
    static void printArr(Integer[] arr){
        for(int i=0; i<arr.length-1; i++)
            pw.print(arr[i]+" ");
        pw.println(arr[arr.length-1]);
    }
    static class Scanner {
        StringTokenizer st;
        BufferedReader br;
        public Scanner(InputStream s) {
            br = new BufferedReader(new InputStreamReader(s));
        }

        public Scanner(FileReader r) {
            br = new BufferedReader(r);
        }

        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 int[] nextArr(int n) throws IOException {
            int[] arr = new int[n];
            for (int i = 0; i < arr.length; i++)
                arr[i] = nextInt();
            return arr;
        }
        public Integer[] nextsort(int n) throws IOException{
            Integer[] arr=new Integer[n];
            for(int i=0; i<n; i++)
                arr[i]=nextInt();
            return arr;
        }
        public Pair nextPair() throws IOException{
            return new Pair(nextInt(), nextInt());
        }
        public Pair[] nextPairArr(int n, int l) throws IOException{
            Pair[] arr=new Pair[n];
            if(l==2){
                int[] a=nextArr(n), b=nextArr(n);
                for(int i=0; i<n; i++)
                    arr[i]=new Pair(a[i], b[i]);
                return arr;
            }
            for(int i=0; i<n; i++)
                arr[i]=new Pair(nextInt(), nextInt());
            return arr;
        }
        public boolean ready() throws IOException {
            return br.ready();
        }
    }
    static class Pair implements Comparable<Pair>{
        int x;
        int y;
        public Pair(int x, int y) {
            this.x=x;
            this.y=y;
        }
        public int hashCode() {
            return this.x*2000+this.y;
        }
        public int compareTo(Pair other) {
            if(this.y!=other.y)
                return this.y-other.y;
            return this.x-other.x;
        }
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (this.getClass() != obj.getClass()) {
                return false;
            }
            Pair p = (Pair) obj;
            return this.x==p.x && this.y==p.y;
        }
        public boolean equal(Pair p){
            if(this.x==p.x)
                return this.y==p.y;
            if(this.x==p.y)
                return this.y==p.x;
            else
                return false;
        }
        public String toString(){
            return this.x+" "+this.y;
        }
    }
    static class Triple implements Comparable<Triple>{
        int x;
        int y;
        char c;
        public Triple(int x, int y, char c){
            this.x=x;
            this.y=y;
            this.c=c;
        }
        public int compareTo(Triple t){
            return this.y-t.y;
        }
        public String toString(){
            if(this.x==1)
                return "INSERT "+(this.y+1)+" "+this.c;
            else if(this.x==0)
                return "REPLACE "+(this.y+1)+" "+this.c;
            else
                return "DELETE "+(this.y+1);
        }
    }
}
