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
 *
 * @author MaxHeap
 */
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();
      LongPair[] cds = new LongPair[n];
      for (int i = 0; i < n; ++i) {
        cds[i] = Factories.makeLongPair(in.nextInt(), in.nextInt());
      }
      Arrays.sort(cds, Comparator.comparingLong((LongPair e) -> e.second)
          .thenComparing(e -> e.first));

      long ans = 0;
      long sum = 0;
      PriorityQueue<Long> pq = new PriorityQueue<>();
      for (int i = n - 1; i >= 0; --i) {
        sum += cds[i].first;
        pq.add(cds[i].first);
        while (pq.size() > k) {
          sum -= pq.poll();
        }
        ans = Math.max(ans, sum * cds[i].second);
      }
      out.println(ans);
    }

  }

  static class LongPair implements Comparable<LongPair> {

    public long first;
    public long second;

    public LongPair(long first, long second) {
      this.first = first;
      this.second = second;
    }

    public int compareTo(LongPair a) {
      if (second == a.second) {
        return Long.compare(first, a.first);
      }
      return Long.compare(second, a.second);
    }

    public String toString() {
      return "<" + first + ", " + second + ">";
    }

    public boolean equals(Object o) {
      if (this == o) {
        return true;
      }
      if (o == null || getClass() != o.getClass()) {
        return false;
      }

      LongPair longPair = (LongPair) o;

      if (first != longPair.first) {
        return false;
      }
      return second == longPair.second;
    }

    public int hashCode() {
      long result = (int) (first ^ (first >>> 32));
      result = 31L * result + (second ^ (second >>> 32));
      return (int) (result >>> 10);
    }

  }

  static interface FastIO {

  }

  static final class Factories {

    private Factories() {
    }

    public static LongPair makeLongPair(long first, long second) {
      return new LongPair(
          first, second);
    }

  }

  static class InputReader implements FastIO {

    private InputStream stream;
    private static final int DEFAULT_BUFFER_SIZE = 1 << 16;
    private static final int EOF = -1;
    private byte[] buf = new byte[DEFAULT_BUFFER_SIZE];
    private int curChar;
    private int numChars;

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

    public int read() {
      if (this.numChars == EOF) {
        throw new UnknownError();
      } else {
        if (this.curChar >= this.numChars) {
          this.curChar = 0;

          try {
            this.numChars = this.stream.read(this.buf);
          } catch (IOException ex) {
            throw new InputMismatchException();
          }

          if (this.numChars <= 0) {
            return EOF;
          }
        }

        return this.buf[this.curChar++];
      }
    }

    public int nextInt() {
      int c;
      for (c = this.read(); isSpaceChar(c); c = this.read()) {
      }

      byte sgn = 1;
      if (c == 45) {
        sgn = -1;
        c = this.read();
      }

      int res = 0;

      while (c >= 48 && c <= 57) {
        res *= 10;
        res += c - 48;
        c = this.read();
        if (isSpaceChar(c)) {
          return res * sgn;
        }
      }

      throw new InputMismatchException();
    }

    public static boolean isSpaceChar(int c) {
      return c == 32 || c == 10 || c == 13 || c == 9 || c == EOF;
    }

  }
}

