import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.Comparator;
import java.util.Collections;
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);
        OutputWriter out = new OutputWriter(outputStream);
        TaskC solver = new TaskC();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskC {
        static int[] t;
        static int[] b;

        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.nextInt(), k = in.nextInt();
            t = new int[n + 1];
            b = new int[n + 1];
            int[] indices = new int[n + 1];
            ArrayList<intWrapper> perm = new ArrayList<>();
            ArrayList<intWrapper> perm2 = new ArrayList<>();
            for (int i = 1; i <= n; i++) {
                t[i] = in.nextInt();
                b[i] = in.nextInt();
                perm.add(new intWrapper(i));
                perm2.add(new intWrapper(i));
            }
            Collections.shuffle(perm);
            Collections.sort(perm, new TaskC.intComparator());
            Collections.shuffle(perm2);
            Collections.sort(perm2, new TaskC.intComparator2());
            for (int i = 1; i <= n; i++) {
                indices[perm2.get(i - 1)._ind] = i;
            }
            TaskC.segtree tree = new TaskC.segtree(n);
            long ans = 0;
            for (int i = n - 1; i >= 0; i--) {
                int ind = perm.get(i)._ind;
                tree.update(1, n, indices[ind], t[ind], 1);
                ans = Long.max(ans, tree.getK(1, n, Integer.min(n - i, k), 1) * (new Long(b[ind])));
            }
            out.println(ans);
        }

        public class intWrapper {
            int _ind;

            public intWrapper() {

            }

            public intWrapper(int v) {
                _ind = v;
            }

        }

        static class intComparator implements Comparator<intWrapper> {
            public int compare(intWrapper x, intWrapper y) {
                return (new Integer(b[x._ind])).compareTo(new Integer(b[y._ind]));
            }

        }

        static class intComparator2 implements Comparator<intWrapper> {
            public int compare(intWrapper x, intWrapper y) {
                return (new Integer(t[x._ind])).compareTo(new Integer(t[y._ind]));
            }

        }

        static class segtree {
            long[] st;
            int[] count;

            segtree(int n) {
                st = new long[4 * n + 10];
                count = new int[4 * n + 10];
            }

            void update(int s, int e, int i, int v, int ind) {
                if (s > i || e < i) return;
                count[ind]++;
                st[ind] += v;
                if (s == e) return;
                int mid = (s + e) / 2;
                if (i <= mid) update(s, mid, i, v, ind * 2);
                else update(mid + 1, e, i, v, ind * 2 + 1);
            }

            long getK(int s, int e, int k, int ind) {
                if (k == 0) return 0;
                if (s == e) return st[ind];
                int mid = (s + e) / 2;
                if (count[2 * ind + 1] >= k) return getK(mid + 1, e, k, 2 * ind + 1);
                return st[2 * ind + 1] + getK(s, mid, k - count[2 * ind + 1], 2 * ind);
            }

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void println(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
            writer.print('\n');
        }

        public void close() {
            writer.close();
        }

    }

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

        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 boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            boolean isSpaceChar(int ch);

        }

    }
}

