import java.io.*;
import java.util.*;

public class A {

    public static void main(String args[]) throws Exception {
        int n = scn.nextInt();
        int k = scn.nextInt();
        Pair[] arr = new Pair[n];
        for(int i=0; i<n; i++){
            long t = scn.nextLong();
            long b = scn.nextLong();
            arr[i] = new Pair(t, b, i);
        }

        Arrays.sort(arr, new Comparator<Pair>() {
            @Override
            public int compare(Pair o1, Pair o2) {
                return Long.compare(o1.b, o2.b);
            }
        });
        long sum = 0, count = 0;
        int i;
        PriorityQueue<Long> pq = new PriorityQueue<>();
        long[] max = new long[n];
        for(i=n-1; i>=0 && count<k-1; i--, count++){
            max[i] = sum;
            pq.add(arr[i].t);
            sum += arr[i].t;
        }

        for(; i>=0; i--){
            if(pq.isEmpty()){
//                max[i] = arr[i].t;
                continue;
            }
            max[i] = sum;
            if(pq.peek()<arr[i].t){
                sum -= pq.poll();
                sum += arr[i].t;
                pq.add(arr[i].t);
            }
        }

        long res = Integer.MIN_VALUE;

        for(i=0; i<n; i++){
            res = Math.max(res, arr[i].b*(max[i]+arr[i].t));
        }

        out.println(res);
        out.close();
    }

    static class Pair {
        long t, b, i;
        Pair(long x, long y, int i) {
            this.t = x;
            this.b = y;
            this.i = i;
        }
        @Override
        public String toString() {
            return this.t + " [" + this.b + "]";
        }
    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private 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 String nextLine() {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

        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;
        }

        public double nextDouble() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, nextInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, nextInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            }
            while (!isSpaceChar(c));

            return res.toString();
        }

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

        public String next() {
            return readString();
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }
    }

    public static InputReader scn = new InputReader(System.in);
    public static PrintWriter out = new PrintWriter(System.out);
}