



import java.util.*;
import java.lang.*;
import java.lang.reflect.Array;
import java.io.*;
import java.math.*;
import java.text.DecimalFormat;


public class Prac{   

 
    static class InputReader {
 
		private final InputStream stream;
		private final byte[] buf = new byte[8192];
		private int curChar, snumChars;
 
		public InputReader(InputStream st) {
			this.stream = st;
		}
 
		public int read() {
			if (snumChars == -1)
	 			throw new InputMismatchException();
			if (curChar >= snumChars) {
				curChar = 0;
				try {
					snumChars = stream.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
 
		public int ni() {
			int c = read();
			while (isSpaceChar(c)) {
				c = read();
			}
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			int res = 0;
			do {
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public long nl() {
			int c = read();
			while (isSpaceChar(c)) {
				c = read();
			}
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			long res = 0;
			do {
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public int[] nia(int n) {
			int a[] = new int[n];
			for (int i = 0; i < n; i++) {
				a[i] = ni();
			}
			return a;
		}
 
		public String rs() {
			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 String nextLine() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndOfLine(c));
			return res.toString();
		}
 
		public boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
 
		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
 
	}
    static PrintWriter w = new PrintWriter(System.out);
    public static void main(String[] args) throws IOException {   
        InputReader sc=new InputReader(System.in);
        
       int n=sc.ni();
       int k=sc.ni();
       int arr[][]=new int[n][2];
       
       for(int i=0;i<n;i++){
           arr[i][0]=sc.ni();
           arr[i][1]=sc.ni();
       }
      //twoDSort(arr,0,n-1,1);
      Arrays.sort(arr,Comparator.comparing((int arr1[])->arr1[1]).reversed());
       long maxsum=0,ans=Long.MIN_VALUE;
       PriorityQueue<Integer> p=new PriorityQueue<>();
      for(int i=0;i<n;i++){
            maxsum+=(long)arr[i][0];
            p.add(arr[i][0]);
            
            while(p.size()>k){
                int a=(int)p.remove();
                maxsum-=(long)a;
            }
          
//            if(arr[i][0]<(int)p.peek()){
//                long suma=maxsum-(long)p.peek()+(long)arr[i][0];
//                
//                ans=Math.max(ans,suma*(long)arr[i][1]);
//               
//            }
//            else{
//                ans=Math.max(ans,maxsum*(long)arr[i][1]);
//            }
            long x=maxsum*(long)arr[i][1];
            ans=Math.max(ans,x);
       }
       w.println(ans);
        w.close();
    }
}
