挑战程序竞赛系列（94）：3.6凸包（5）

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Map;
import java.util.StringTokenizer;

public class Main{

    String INPUT = "./data/judge/201710/P2079.txt";

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    class P implements Comparable<P>{
        int x;
        int y;

        P(int x, int y){
            this.x = x;
            this.y = y;
        }

        P sub(P a) {
            return new P(x - a.x, y - a.y);
        }

        int det(P a) {
            return x * a.y - y * a.x;
        }

        @Override
        public int compareTo(P o) {
            return x - o.x != 0 ? x - o.x : y - o.y;
        }

    }

    int N;
    P[] p;

    double area(P a, P b, P c) {
        double ans = (c.x - a.x) * (b.y - a.y) - (c.y - a.y) * (b.x - a.x);
        return 0.5 * Math.abs(ans);
    }

    P[] convexHull() {
        Arrays.sort(p);
        P[] qs = new P[2 * N];
        int k = 0;
        for (int i = 0; i < N; ++i) {
            while (k > 1 && qs[k - 1].sub(qs[k - 2]).det(p[i].sub(qs[k - 2])) < 0) k --;
            qs[k++] = p[i];
        }

        for (int i = N - 2, t = k; i >= 0; --i) {
            while (k > t && qs[k - 1].sub(qs[k - 2]).det(p[i].sub(qs[k - 2])) < 0) k--;
            qs[k++] = p[i];
        }

        P[] res = new P[k - 1];
        System.arraycopy(qs, 0, res, 0, k - 1);
        return res;
    }

    void solve() {
        P[] qs = convexHull();
        int n = qs.length;
        double ans = 0;

        for (int offset = 1; offset < (n + 1) / 2; ++offset) {
            int fir = 0;
            int pos = (fir + offset + 1) % n;
            do {
                int sec = (fir + offset) % n;
                while ((pos + 1) % n != fir && area(qs[fir % n], qs[sec % n], qs[(pos + 1) % n]) 
                           >= area(qs[fir % n], qs[sec % n], qs[pos % n])) {
                    pos = (pos + 1) % n;
                }
                ans = Math.max(ans, area(qs[fir % n], qs[sec % n], qs[pos % n]));
                fir = (fir + 1) % n;
            }
            while (fir != 0);
        }

        out.printf("%.2f\n", ans);
    }

    void read() {
        while (true) {
            N = ni();
            if (N == -1) break;
            p = new P[N];

            for (int i = 0; i < N; ++i) {
                p[i] = new P(ni(), ni());
            }

            solve();
        }
    }

    FastScanner in;
    PrintWriter out;

    void run() throws IOException {
        boolean oj;
        try {
            oj = ! System.getProperty("user.dir").equals("F:\\oxygen_workspace\\Algorithm");
        } catch (Exception e) {
            oj = System.getProperty("ONLINE_JUDGE") != null;
        }

        InputStream is = oj ? System.in : new FileInputStream(new File(INPUT));
        in = new FastScanner(is);
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        read();
        out.flush();
        if (!oj){
            System.out.println("[" + (System.currentTimeMillis() - s) + "ms]");
        }
    }

    public boolean more(){
        return in.hasNext();
    }

    public int ni(){
        return in.nextInt();
    }

    public long nl(){
        return in.nextLong();
    }

    public double nd(){
        return in.nextDouble();
    }

    public String ns(){
        return in.nextString();
    }

    public char nc(){
        return in.nextChar();
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        boolean hasNext;

        public FastScanner(InputStream is) throws IOException {
            br = new BufferedReader(new InputStreamReader(is));
            hasNext = true;
        }

        public String nextToken() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    hasNext = false;
                    return "##";
                }
            }
            return st.nextToken();
        }

        String next = null;
        public boolean hasNext(){
            next = nextToken();
            return hasNext;
        }

        public int nextInt() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Integer.parseInt(more);
        }

        public long nextLong() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Long.parseLong(more);
        }

        public double nextDouble() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Double.parseDouble(more);
        }

        public String nextString(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more;
        }

        public char nextChar(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more.charAt(0);
        }
    }
}