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

public class Main1 {

	static FastReader input = new FastReader();
	static PrintWriter out = new PrintWriter(System.out);
	static int n, k;
	static Pair a[];
	static long prefix[];

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

		n = input.nextInt();
		k = input.nextInt();

		a = new Pair[n];

		for (int i = 0; i < n; i++) {
			int t = input.nextInt();
			int b = input.nextInt();
			a[i] = new Pair(b, t);
		}
		Arrays.sort(a);

	
		PriorityQueue<Integer> pq = new PriorityQueue<>();
		long sum = 0;
		long max = 0;

		for (int i = n - 1; i >= 0; i--) {
			pq.add(a[i].y);
			sum += (long) a[i].y;
			if (pq.size() == k) {
				long cur = sum * (long) a[i].x;
				max = Math.max(max, cur);
				sum -= pq.poll();
			}
		}
		sum = 0;
		for (int i = n - 1; i > n - k; i--) {
			sum += (long) a[i].y;
			long cur = sum * (long) a[i].x;
			max = Math.max(max, cur);
		}
		out.println(max);
		out.flush();
	}

	static class FastReader {
		BufferedReader br;
		StringTokenizer st;

		public FastReader() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() throws IOException {
			while (st == null || !st.hasMoreElements()) {
				st = new StringTokenizer(br.readLine());
			}
			return st.nextToken();
		}

		int nextInt() throws NumberFormatException, IOException {
			return Integer.parseInt(next());
		}

		long nextLong() throws NumberFormatException, IOException {
			return Long.parseLong(next());
		}

		double nextDouble() throws NumberFormatException, IOException {
			return Double.parseDouble(next());
		}

		String nextLine() throws IOException {
			String str = "";
			str = br.readLine();
			return str;
		}
	}

	static class con {
		static int IINF = (int) 1e9;
		static int _IINF = (int) -1e9;
		static long LINF = (long) 1e15;
		static long _LINF = (long) -1e15;
		static double EPS = 1e-9;
	}

	static class Triple implements Comparable<Triple> {
		int x;
		int y;
		int z;

		Triple(int x, int y, int z) {
			this.x = x;
			this.y = y;
			this.z = z;
		}

		@Override
		public int compareTo(Triple o) {
			if (x == o.x && y == o.y)
				return z - o.z;
			if (x == o.x)
				return y - o.y;
			return x - o.x;
		}
	}

	static class Pair implements Comparable<Pair> {
		int x;
		int y;

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

		}

		@Override
		public int compareTo(Pair o) {
			if (x == o.x)
				return y - o.y;
			return x - o.x;

		}

		@Override
		public String toString() {

			return "(" + x + ", " + y + ")";
		}

	}

	static void shuffle(int[] a) {
		for (int i = 0; i < a.length; i++) {
			int r = i + (int) (Math.random() * (a.length - i));
			int tmp = a[r];
			a[r] = a[i];
			a[i] = tmp;
		}
	}

	static int gcd(int a, int b) {
		return b == 0 ? a : gcd(b, a % b);
	}

}