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

public class C {
    public class Song implements Comparable {
        long t;
        long b;

        public Song(long t, long b) {
            this.t = t;
            this.b = b;
        }

        @Override
        public int compareTo(Object o) {
            Song c = (Song) o;
            if (this.b == c.b) {
                return (int) -(this.t - c.t);
            }
            return (int) -(this.b - c.b);
        }
    }

    public void solve() throws IOException {
        PriorityQueue<Long> q = new PriorityQueue<>();
        int n = nextInt();
        int k = nextInt();
        Song[] s = new Song[n];
        for (int i = 0; i < n; i++) {
            s[i] = new Song(nextLong(), nextLong());
        }
        Arrays.sort(s);
        long[] sb = new long[n + 1];
        for (int i = 0; i < n; i++) {
            sb[i + 1] = sb[i] + s[i].t;
        }
        long ans = 0;
        long summ = s[0].t;
        q.add(s[0].t);
        ans = s[0].t * s[0].b;
        for (int i = 1; i < n; i++) {
            if (q.size() < k) {
                summ += s[i].t;
                q.add(s[i].t);
            } else {
                long del = q.poll();
                summ -= del;
                del = Math.max(del, s[i].t);
                q.add(del);
                summ += del;
            }
            long cur = (summ) * s[i].b;
            ans = Math.max(cur, ans);
        }
        System.out.println(ans);
    }

    public void run() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);

            solve();

            out.close();
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    BufferedReader br;
    StringTokenizer in;
    PrintWriter out;

    public String nextToken() throws IOException {
        while (in == null || !in.hasMoreTokens()) {
            in = new StringTokenizer(br.readLine());
        }
        return in.nextToken();
    }

    public int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

    public double nextDouble() throws IOException {
        return Double.parseDouble(nextToken());
    }

    public long nextLong() throws IOException {
        return Long.parseLong(nextToken());
    }

    public int[] nextArr(int n) throws IOException {
        int[] res = new int[n];
        for (int i = 0; i < n; i++) {
            res[i] = nextInt();
        }
        return res;
    }

    public static void main(String[] args) throws IOException {
        Locale.setDefault(Locale.US);
        new C().run();
    }
}