
import java.io.*;

import java.util.*;

import javax.swing.text.InternationalFormatter;

import java.math.*;

public class template {

	static long time = 0, mp = Integer.MAX_VALUE, k = 0, cnt = 0, edge = 0, no = 0;

	static boolean bul = false;
	static int[] ht;
	static long[] dis;
	static boolean[][] b;
	static char[][] g;
	static String sb = "";
	static ArrayList<Integer>[] a;
	static HashSet<Long>[] c;
	public static PrintWriter out;

	static HashMap<Integer, HashSet<Integer>> hm = new HashMap<>();

	static ArrayList<Long> p = new ArrayList<>();

	public static void main(String[] args) throws IOException {

		Reader scn = new Reader();

		int n = scn.nextInt(), k = scn.nextInt();

		PriorityQueue<pair> pq = new PriorityQueue<>();

		ArrayList<pair> p = new ArrayList<>();

		for (int i = 0; i < n; i++) {
			p.add(new pair(scn.nextInt(), scn.nextInt()));
		}

		Collections.sort(p, new comp());

		// for(pair t:p)
		// System.out.println(t.t+" "+t.b);
		//

		long ans = Integer.MIN_VALUE, ktime=0;
		
		for (int i = 0; i < p.size(); i++) {


			long beaudy = p.get(i).b;
			long time = p.get(i).t;

			ans = Math.max(ans, (long)(beaudy * (time + ktime)));
			
			 if(pq.size() < k-1){
				 pq.add(p.get(i));
				 ktime+=p.get(i).t;
			 }
			 
			 else{
				     if(pq.size()!=0 && pq.peek().t <p.get(i).t){
				    	 ktime-=pq.remove().t;
				    	 ktime+=p.get(i).t;
				    	 pq.add(p.get(i));
				     }
			 }

		}

		System.out.println(ans);
	}

	// _________________________TEMPLATE_____________________________________________________________

	static class comp implements Comparator<pair> {

		@Override
		public int compare(pair o1, pair o2) {
			return o2.b - o1.b;
		}

	}

	static class pair implements Comparable<pair> {
		int t;
		int b;
		int d;

		pair(int a, int b) {
			this.t = a;
			this.b = b;
			this.d = a - b;
		}

		@Override
		public int compareTo(pair o) {

			return (int) (this.t - o.t);

		}
	}

	public static class Reader {
		final private int BUFFER_SIZE = 1 << 16;
		private DataInputStream din;
		private byte[] buffer;
		private int bufferPointer, bytesRead;

		public Reader() {
			din = new DataInputStream(System.in);
			buffer = new byte[BUFFER_SIZE];
			bufferPointer = bytesRead = 0;
		}

		public Reader(String file_name) throws IOException {
			din = new DataInputStream(new FileInputStream(file_name));
			buffer = new byte[BUFFER_SIZE];
			bufferPointer = bytesRead = 0;
		}

		public String readLine() throws IOException {
			byte[] buf = new byte[100000 + 1]; // line length
			int cnt = 0, c;
			while ((c = read()) != -1) {
				if (c == '\n')
					break;
				buf[cnt++] = (byte) c;
			}
			return new String(buf, 0, cnt);
		}

		public int nextInt() throws IOException {
			int ret = 0;
			byte c = read();
			while (c <= ' ')
				c = read();
			boolean neg = (c == '-');
			if (neg)
				c = read();
			do {
				ret = ret * 10 + c - '0';
			} while ((c = read()) >= '0' && c <= '9');

			if (neg)
				return -ret;
			return ret;
		}

		public long nextLong() throws IOException {
			long ret = 0;
			byte c = read();
			while (c <= ' ')
				c = read();
			boolean neg = (c == '-');
			if (neg)
				c = read();
			do {
				ret = ret * 10 + c - '0';
			} while ((c = read()) >= '0' && c <= '9');
			if (neg)
				return -ret;
			return ret;
		}

		public double nextDouble() throws IOException {
			double ret = 0, div = 1;
			byte c = read();
			while (c <= ' ')
				c = read();
			boolean neg = (c == '-');
			if (neg)
				c = read();

			do {
				ret = ret * 10 + c - '0';
			} while ((c = read()) >= '0' && c <= '9');

			if (c == '.') {
				while ((c = read()) >= '0' && c <= '9') {
					ret += (c - '0') / (div *= 10);
				}
			}

			if (neg)
				return -ret;
			return ret;
		}

		private void fillBuffer() throws IOException {
			bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
			if (bytesRead == -1)
				buffer[0] = -1;
		}

		private byte read() throws IOException {
			if (bufferPointer == bytesRead)
				fillBuffer();
			return buffer[bufferPointer++];
		}

		public void close() throws IOException {
			if (din == null)
				return;
			din.close();
		}

		public int[] nextIntArray(int n) throws IOException {
			int[] arr = new int[n];
			for (int i = 0; i < n; i++) {
				arr[i] = nextInt();
			}
			return arr;
		}

		public long[] nextLongArray(int n) throws IOException {
			long[] arr = new long[n];
			for (int i = 0; i < n; i++) {
				arr[i] = nextLong();
			}
			return arr;
		}

		public int[][] nextInt2DArray(int m, int n) throws IOException {
			int[][] arr = new int[m][n];
			for (int i = 0; i < m; i++) {
				for (int j = 0; j < n; j++)
					arr[i][j] = nextInt();
			}
			return arr;
		}

	}

}
