
import javafx.scene.CacheHint;

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

public class Q3 {


    public static void main(String[] args) {

        InputReader in = new InputReader();
        int N = in.nextInt(), K = in.nextInt();

        int input[][] = new int[N][2];

        for (int i = 1; i <= N; i++) {
            input[i - 1][0] = in.nextInt();
            input[i - 1][1] = in.nextInt();
        }

        Arrays.sort(input, new Comparator<int[]>() {

            public int compare(int[] a, int[] b) {
                 int temp=Integer.compare(b[1], a[1]);
                 //if(temp==0)
                     //return Integer.compare(a[0],b[0]);

                 return temp;
            }
        });

        PriorityQueue<Integer> pq=new PriorityQueue();
        long add=0,max=Integer.MIN_VALUE;
        for (int i = 0; i < N; i++) {

            add+=input[i][0];
            pq.add(input[i][0]);

            if(pq.size()>K){
                add-=pq.poll();
            }


            max=Math.max(add*input[i][1],max);

        }

        System.out.println(max);
    }


        static class InputReader {
            public BufferedReader reader;
            public StringTokenizer tokenizer;

            public int[] shuffle(int[] arr) {
                Random r = new Random();
                for (int i = 1, j; i < arr.length; i++) {
                    j = r.nextInt(i);
                    arr[i] = arr[i] ^ arr[j];
                    arr[j] = arr[i] ^ arr[j];
                    arr[i] = arr[i] ^ arr[j];
                }
                return arr;
            }

            public InputReader() {
                reader = new BufferedReader(new InputStreamReader(System.in), 32768);
                tokenizer = null;
            }

            public InputReader(InputStream stream) {
                reader = new BufferedReader(new InputStreamReader(System.in), 32768);
                tokenizer = null;
            }

            public String next() {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    try {
                        tokenizer = new StringTokenizer(reader.readLine());
                    } catch (IOException e) {
                        throw new RuntimeException(e);
                    }
                }
                return tokenizer.nextToken();
            }

            public char nextChar() {
                return next().charAt(0);
            }

            public int nextInt() {
                return Integer.parseInt(next());
            }

            public long nextLong() {
                return Long.parseLong(next());
            }

            public double nextDouble() {
                return Double.parseDouble(next());
            }

            public int[] nextIntArr(int n) {
                int[] arr = new int[n];
                for (int i = 0; i < n; i++) {
                    arr[i] = this.nextInt();
                }
                return arr;
            }

            public Integer[] nextIntegerArr(int n) {
                Integer[] arr = new Integer[n];
                for (int i = 0; i < n; i++)
                    arr[i] = new Integer(this.nextInt());

                return arr;
            }

            public int[][] next2DIntArr(int n, int m) {
                int[][] arr = new int[n][m];
                for (int i = 0; i < n; i++) {
                    for (int j = 0; j < m; j++) {
                        arr[i][j] = this.nextInt();
                    }
                }
                return arr;
            }

            public int[] nextSortedIntArr(int n) {
                int[] arr = new int[n];
                for (int i = 0; i < n; i++) {
                    arr[i] = this.nextInt();
                }
                Arrays.sort(arr);
                return arr;
            }

            public long[] nextLongArr(int n) {
                long[] arr = new long[n];
                for (int i = 0; i < n; i++) {
                    arr[i] = this.nextLong();
                }
                return arr;
            }

            public char[] nextCharArr(int n) {
                char[] arr = new char[n];
                for (int i = 0; i < n; i++) {
                    arr[i] = this.nextChar();
                }
                return arr;
            }


            public static int gcd(int a, int b) {
                return b == 0 ? a : gcd(b, a % b);
            }

            public static int[] uwiSieve(int n) {
                if (n <= 32) {
                    int[] primes = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31};
                    for (int i = 0; i < primes.length; i++) {
                        if (n < primes[i]) {
                            return Arrays.copyOf(primes, i);
                        }
                    }
                    return primes;
                }

                int u = n + 32;
                double lu = Math.log(u);
                int[] ret = new int[(int) (u / lu + u / lu / lu * 1.5)];
                ret[0] = 2;
                int pos = 1;

                int[] isp = new int[(n + 1) / 32 / 2 + 1];
                int sup = (n + 1) / 32 / 2 + 1;

                int[] tprimes = {3, 5, 7, 11, 13, 17, 19, 23, 29, 31};
                for (int tp : tprimes) {
                    ret[pos++] = tp;
                    int[] ptn = new int[tp];
                    for (int i = (tp - 3) / 2; i < tp << 5; i += tp)
                        ptn[i >> 5] |= 1 << (i & 31);
                    for (int i = 0; i < tp; i++) {
                        for (int j = i; j < sup; j += tp)
                            isp[j] |= ptn[i];
                    }
                }

                // 3,5,7
                // 2x+3=n
                int[] magic = {0, 1, 23, 2, 29, 24, 19, 3, 30, 27, 25, 11, 20, 8, 4, 13, 31, 22, 28, 18, 26, 10, 7, 12, 21, 17,
                        9, 6, 16, 5, 15, 14};
                int h = n / 2;
                for (int i = 0; i < sup; i++) {
                    for (int j = ~isp[i]; j != 0; j &= j - 1) {
                        int pp = i << 5 | magic[(j & -j) * 0x076be629 >>> 27];
                        int p = 2 * pp + 3;
                        if (p > n)
                            break;
                        ret[pos++] = p;
                        for (int q = pp; q <= h; q += p)
                            isp[q >> 5] |= 1 << (q & 31);
                    }
                }

                return Arrays.copyOf(ret, pos);
            }

            public static int[] radixSort(int[] f) {
                return radixSort(f, f.length);
            }

            public static int[] radixSort(int[] f, int n) {
                int[] to = new int[n];
                {
                    int[] b = new int[65537];
                    for (int i = 0; i < n; i++) b[1 + (f[i] & 0xffff)]++;
                    for (int i = 1; i <= 65536; i++) b[i] += b[i - 1];
                    for (int i = 0; i < n; i++) to[b[f[i] & 0xffff]++] = f[i];
                    int[] d = f;
                    f = to;
                    to = d;
                }
                {
                    int[] b = new int[65537];
                    for (int i = 0; i < n; i++) b[1 + (f[i] >>> 16)]++;
                    for (int i = 1; i <= 65536; i++) b[i] += b[i - 1];
                    for (int i = 0; i < n; i++) to[b[f[i] >>> 16]++] = f[i];
                    int[] d = f;
                    f = to;
                    to = d;
                }
                return f;
            }
        }


    }


// for(Map.Entry<Integer,Integer> Test: map.entrySet()){
//            int Key=Test.getKey();
//            int val=Test.getValue();
//        }


//        HashMap<Integer,Integer> map=new HashMap<>();
//        for(int i=0;i<;i++){
//             ch=input[i];
//        if(map.containsKey(ch))
//            map.put(ch,map.get(ch)+1);
//        else
//            map.put(ch,1);
//        }
//
//
