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

public class Solu {
    static StringBuffer str = new StringBuffer();
    static InputReader in = new InputReader(System.in);
    static int mm = 1000000007;
    static int n;
    static ArrayList<Integer> tree[];
    static boolean vis[];
    static long dp[][];
    static long time[];
    static long beu[];
    public static void main(String[] args) {
        int t, i, j, tt, m, k;
        t = 1;
        for (tt = 0; tt < t; tt++) {
            n=in.nextInt();
            k=in.nextInt();
            Entity[] en=new Entity[n];
            for(i=0;i<n;i++){
                int l=in.nextInt();
                int r=in.nextInt();
                en[i]=new Entity(r,l);
            }
            Arrays.sort(en,com);
            long maxs[]=new long[n];
            PriorityQueue<Integer> pq=new PriorityQueue<>();
            long sum=0;
            int cc=1;
            int last=en[n-1].c;
            for(i=n-2;i>=0;i--){
                if(cc==k){
                    if(pq.size()==0)
                        continue;
                    if(pq.peek()>=last){
                        maxs[i]=sum;
                    }else{
                        sum-=pq.peek();
                        pq.poll();
                        sum+=last;
                        maxs[i] = sum;
                        pq.add(last);
                    }
                }else{
                    pq.add(last);
                    sum+=last;
                    maxs[i] = sum;
                    cc++;
                }
                last=en[i].c;
            }
            long ans = 0;
            for (i = 0; i < n; i++) {
                long gh = (long)en[i].a*(maxs[i] +en[i].c);
                ans=Math.max(gh,ans);
            }
            ap(ans+"\n");
        }

        pn();
    }

    static void initT(int n) {
        vis = new boolean[n];
        tree = new ArrayList[n];
        for (int i = 0; i < n; i++) {
            tree[i] = new ArrayList<>();
        }
    }

    static class hog {

        char ch;
    }

    static final Comparator<Entity> com = new Comparator<Entity>() {
        public int compare(Entity x, Entity y) {
            int xx = Integer.compare(x.a, y.a);
            if (xx == 0) {
                return Long.compare(y.c, x.c);
            } else {
                return xx;
            }
        }
    };

    static class Pair {

        int a, b, c;

        Pair(int x, int y, int z) {
            a = x;
            b = y;
            c = z;
        }
    }

    static class Entity {

        int a;
        int c;

        Entity(int p, int q) {
            a = p;
            c = q;
        }
    }

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

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

    static long power(long x, int y, int m) {
        if (y == 0) {
            return 1;
        }
        long p = power(x, y / 2, m) % m;
        p = (p * p) % m;
        return (y % 2 == 0) ? p : (x * p) % m;
    }

    public static void ap(String st) {
        str.append(st);
    }

    public static void pn() {
        System.out.println(str);
    }

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

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

        public void skip(int x) {
            while (x-- > 0) {
                read();
            }
        }

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

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

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

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

            return res.toString();
        }

        public String nextLine() {
            StringBuffer buf = new StringBuffer();
            int c = read();
            while (c != '\n' && c != -1) {
                if (c != '\r') {
                    buf.appendCodePoint(c);
                }
                c = read();
            }
            return buf.toString();
        }


        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 boolean hasNext() {
            int value;
            while (isSpaceChar(value = peek()) && value != -1) {
                read();
            }
            return value != -1;
        }

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