import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.util.function.Function;

public class Main {

    public static void main(String[] args) {

        Parser parser = new Parser();
        int t = 1;
        for (int i = 1; i <= t; i++) {
            Problem problem = new Problem();
            problem.solve(i);
        }
    }
}

class Problem {
    Parser parser = new Parser();

    public void solve(int testNum) {
        int n = parser.parseInt();
        int k = parser.parseInt();

        List<Pair> pairs = new ArrayList<>();
        for(int i = 0; i < n; i++) {
            pairs.add(new Pair(parser.parseInt(), parser.parseInt()));
        }

        pairs.sort(Comparator.comparing(Pair::getB, Comparator.reverseOrder())
        .thenComparing(Pair::getT, Comparator.reverseOrder()));

        long ans = 0;
        long sum = 0;
        PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();
        for(int i = 0; i < k; i++) {
            Pair pair = pairs.get(i);
            priorityQueue.add(pair.t);
            sum += pair.t;
            ans = Math.max(ans, sum * pair.b);
        }

        for(int i = k; i < n; i++) {
            Pair pair = pairs.get(i);
            sum -= priorityQueue.poll();
            sum += pair.t;
            priorityQueue.add(pair.t);
            ans = Math.max(ans, sum * pair.b);
        }
        System.out.println(ans);
    }

    static class Pair {
        int t, b;

        public Pair(int t, int b) {
            this.t = t;
            this.b = b;
        }

        public int getT() {
            return t;
        }

        public int getB() {
            return b;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Pair pair = (Pair) o;
            return t == pair.t &&
                    b == pair.b;
        }

        @Override
        public int hashCode() {
            return Objects.hash(t, b);
        }

        @Override
        public String toString() {
            return "Pair{" +
                    "t=" + t +
                    ", b=" + b +
                    '}';
        }
    }
}

class MathUtil {

    public static List<Integer> getPrimes(int r) {
        if (r < 2) {
            return List.of();
        }

        List<Integer> result = new ArrayList<>();
        result.add(2);
        boolean[] is = new boolean[r + 1];
        for (int i = 4; i <= r; i += 2) {
            is[i] = true;
        }
        for (int i = 3; i <= r; i += 2) {
            if (is[i]) continue;
            result.add(i);
            for (long j = (long) i * i; j <= r; j += i) {
                is[(int) j] = true;
            }
        }
        return result;
    }

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

    public static long pow(long x, long y) {
        return pow(x, y, Function.identity());
    }

    public static long pow(long x, long y, long mod) {
        return pow(x, y, v -> v % mod);
    }

    private static long pow(long x, long y, Function<Long, Long> action) {
        long res = 1;
        long t = x;
        while (y > 0) {
            if ((y & 1) == 1) {
                res += t;
                res = action.apply(res);
            }
            t *= t;
            t = action.apply(t);
            y /= 2;
        }
        return res;
    }
}

class DisjointSet {

    private final int[] f;

    public DisjointSet(int size) {
        f = new int[size + 1];
        for (int i = 0; i <= size; i++) {
            f[i] = i;
        }
    }

    public int find(int u) {
        if (f[u] == u) return u;
        return f[u] = find(f[u]);
    }

    public boolean merge(int u, int v) {
        u = find(u);
        v = find(v);
        if (u == v) return false;
        if (u > v) {
            int t = u;
            u = v;
            v = t;
        }
        f[u] = v;
        return true;
    }
}

class Parser {
    private static final BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    private StringTokenizer tokenizer;

    public int parseInt() {
        fill();
        return Integer.parseInt(tokenizer.nextToken());
    }

    public long parseLong() {
        fill();
        return Long.parseLong(tokenizer.nextToken());
    }

    public double parseDouble() {
        fill();
        return Double.parseDouble(tokenizer.nextToken());
    }

    public String parseLine() {
        try {
            return reader.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    String parseString() {
        fill();
        return tokenizer.nextToken();
    }

    private void fill() {
        while (tokenizer == null || !tokenizer.hasMoreElements()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}