import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.StringTokenizer;

public class CF1140C_Playlist {
	static class Scanner {
		BufferedReader br;
		StringTokenizer st;

		public Scanner() {
			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 Song implements Comparable<Song> {
		int length;
		int beauty;

		public Song(int l, int b) {
			length = l;
			beauty = b;
		}

		@Override
		public int compareTo(Song o) {
			// TODO Auto-generated method stub
			return o.beauty - beauty;
		}
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner();
		int n = scanner.nextInt();
		int k = scanner.nextInt();
		Song[] songs = new Song[n];
		for (int i = 0; i < n; i++) {
			songs[i] = new Song(scanner.nextInt(), scanner.nextInt());
		}
		Arrays.sort(songs);
		PriorityQueue<Integer> q = new PriorityQueue<Integer>();
		long sum = 0;
		long ans = 0;
		for (int i = 0; i < n; i++) {
			if (q.size() < k) {
				sum += songs[i].length;
				q.add(songs[i].length);
				ans = Math.max(ans, sum * songs[i].beauty);
			} else {
				int temp = q.peek();
				if (temp < songs[i].length) {
					sum += songs[i].length;
					sum -= temp;
					q.poll();
					q.add(songs[i].length);
					ans = Math.max(ans, sum * songs[i].beauty);
				} else {
					ans = Math.max(ans, (sum - temp + songs[i].length) * songs[i].beauty);
				}
			}
		}
		System.out.println(ans);
	}

}
