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

public class C {

	public static final int MOD = 1000000007;

	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());
		long max = Long.MIN_VALUE;
		long sum = 0;
		long min = Long.MAX_VALUE;
		for(int i=0;i<k;++i){
			min = Math.min(min, p[i].y);
			sum += p[i].x;
			
			if(sum*min > max){
				max = sum*min;
			}
		}
		
		System.out.println(max);
	}

	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;

	}
}
