import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

public class C {

	public static void main(String[] args) {
		FastReader scn = new FastReader();
		int n = scn.nextInt();
		int k = scn.nextInt();
		Pair p[] = new Pair[n];
		for (int i = 0; i < n; ++i) {
			int t = scn.nextInt();
			int b = scn.nextInt();
			p[i] = new Pair(t, b);
		}
		Arrays.sort(p, new SortByB());

		PriorityQueue<Long> q = new PriorityQueue<>(k);
		long sum = 0;
		long ans = 0;
		for (int i = 0; i < n; ++i) {
			Pair e = p[i];
			ans = Math.max(ans, (sum + e.x) * e.y);
			if(k == 1) continue;
			if (q.size() < k - 1) {
				sum += e.x;
				q.add((long) e.x);
			} else {
				if (q.peek() < e.x) {
					sum -= q.poll();
					sum += e.x;
					q.add((long)e.x);
				}
			}
		}
		System.out.println(ans);

	}

	public 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;
		}
	}

}

class SortByB implements Comparator<Pair> {

	@Override
	public int compare(Pair o1, Pair o2) {
		if (o2.y == o1.y) {
			return o2.x - o1.x;
		}
		return o2.y - o1.y;
	}

}

class Pair {
	int x;
	int y;

	Pair(int x, int y) {
		this.x = x;
		this.y = y;

	}
}