import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.Comparator;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CPlaylist solver = new CPlaylist();
        solver.solve(1, in, out);
        out.close();
    }

    static class CPlaylist {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int k = in.nextInt();
            long[][] inf = new long[n][2];
            for (int i = 0; i < n; i++) {
                inf[i][0] = in.nextLong();
                inf[i][1] = in.nextLong();
            }

            Arrays.sort(inf, new Comparator<long[]>() {

                public int compare(long[] o1, long[] o2) {
                    if (o1[1] == o2[1]) return Long.compare(o2[0], o1[0]);
                    return Long.compare(o2[1], o1[1]);
                }
            });

            long ans = 0;
            long mul = Long.MAX_VALUE / 2;
            long lenSum = 0;
            PriorityQueue<Long> P = new PriorityQueue<>((int) 6e5);
            for (int i = 0; i < n; i++) {
                if (P.size() == k) {
                    lenSum -= P.poll();
                    lenSum += inf[i][0];
                    P.add(inf[i][0]);
                    mul = Math.min(inf[i][1], mul);
                } else {
                    lenSum += inf[i][0];
                    P.add(inf[i][0]);
                    mul = Math.min(inf[i][1], mul);
                }
                ans = Math.max(ans, lenSum * mul);
            }
            out.println(ans);
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nextLong() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        private boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

    }
}

