import java.io.*;


import java.util.*;
/*


*/






public class B {
	static FastReader sc=null;
	
	public static void main(String[] args) {
		sc=new FastReader();
		int n=sc.nextInt(),k=sc.nextInt();
		Pair p[]=new Pair[n];
		for(int i=0;i<n;i++)p[i]=new Pair(sc.nextInt(),sc.nextInt(),i);
		Arrays.sort(p);
		PriorityQueue<Integer> pq=new PriorityQueue<>();
		long ans=0;
		long sum=0;
		for(int i=0;i<n;i++) {
			long val=0;
			if(pq.size()>k) {
				int q=pq.remove();
				if(p[i].t<=q)pq.add(q);
				else {
					pq.add(p[i].t);
					sum-=q;
					sum+=p[i].t;
				}
			}
			else {
				pq.add(p[i].t);
				sum+=p[i].t;
			}
			
			val=sum*p[i].b;
			ans=Math.max(ans, val);
		}
		//for(Pair e:p)System.out.print(e.b+" ");
		//System.out.println();
		//print(pre);
		System.out.println(ans);
		
		
	
	
	
	}
	static class Pair implements Comparable<Pair>{
		int t,b,id;
		Pair(int t,int b,int id){
			this.t=t;
			this.b=b;
			this.id=id;
		}
		@Override
		public int compareTo(Pair o) {
			return o.b-this.b;
		}
	}
	
	
	

	
	static int[] reverse(int a[]) {
		ArrayList<Integer> al=new ArrayList<>();
		for(int i:a)al.add(i);
		Collections.sort(al,Collections.reverseOrder());
		for(int i=0;i<a.length;i++)a[i]=al.get(i);
		return a;
	}
	static int gcd(int a,int b) {
		if(b==0)return a;
		else return gcd(b,a%b);
	}
	static long gcd(long a,long b) {
		if(b==0)return a;
		else return gcd(b,a%b);
	}
	
	static int[] ruffleSort(int a[]) {
		ArrayList<Integer> al=new ArrayList<>();
		for(int i:a)al.add(i);
		Collections.sort(al);
		for(int i=0;i<a.length;i++)a[i]=al.get(i);
		return a;
	}
	
	
	
	static void print(int a[]) {
		for(int e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	static void print(boolean a[]) {
		for(boolean e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	static void print(long a[]) {
		for(long e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	static void print(char a[]) {
		for(char e:a) {
			System.out.print(e);
		}
		System.out.println();
	}
	
	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; 
        } 
        int[] readArray(int n) {
    		int a[]=new int [n];
    		for(int i=0;i<n;i++) {
    			a[i]=sc.nextInt();
    		}
    		return a;
    	}
    } 
}






