import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Set;
import java.util.StringTokenizer;
import java.util.stream.Collectors;


public class C{
	
	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; 
        } 
    }
	static class Pair
	{
		long len;
		long beau;
		Pair(long len,long beau)
		{
			this.len = len;
			this.beau = beau;
		}
		
	}
	
	public static void main(String[] args) 
	{
		OutputStream outputStream = System.out;
        FastReader sc = new FastReader();
        PrintWriter out = new PrintWriter(outputStream);
        
        int n = sc.nextInt();
        int k = sc.nextInt();

        Pair p[] = new Pair[n];
        for(int i = 0; i < n; ++i)
            p[i] = new Pair(sc.nextLong(), sc.nextLong());

        Arrays.sort(p, new Comparator<Pair>() {
            public int compare(Pair p1, Pair p2) {
                if(p1.beau > p2.beau) 
                    return -1;
                if(p1.beau < p2.beau)
                    return 1;
                return 0;
            }
        });
        
        
        PriorityQueue<Pair> queue = new PriorityQueue<>(new Comparator<Pair>() {
            public int compare(Pair p1, Pair p2) {
                if(p1.len < p2.len) 
                    return -1;
                if(p1.len > p2.len)
                    return 1;
                return 0;
            }
        });

        long sum = 0, beau = Long.MAX_VALUE;
        long ans = 0;
        for(int i = 0; i < n; ++i) {
            if(queue.size() < k) {
                sum += p[i].len;
                queue.add(p[i]);
            }
            else if(queue.peek().len < p[i].len) {
                Pair curP = queue.poll();
                queue.add(p[i]);
                sum += p[i].len - curP.len;
            }
            beau = p[i].beau;
            ans = Math.max(ans, beau * sum);
        }
        out.println(ans);
        out.close();
	}

}
