import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.stream.IntStream;
import java.util.stream.LongStream;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.io.UncheckedIOException;
import java.util.List;
import java.nio.charset.Charset;
import java.util.StringTokenizer;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.BufferedReader;
import java.util.regex.Pattern;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author mikit
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        LightScanner in = new LightScanner(inputStream);
        LightWriter out = new LightWriter(outputStream);
        CMakeARectangle solver = new CMakeARectangle();
        solver.solve(1, in, out);
        out.close();
    }

    static class CMakeARectangle {
        public void solve(int testNumber, LightScanner in, LightWriter out) {
            Debug.enable("src");
            int n = in.ints();
            long[] a = in.longs(n);

            IntroSort.sort(a);
            List<Long> ans = new ArrayList<>();
            for (int i = n - 2; i >= 0; i--) {
                if (a[i] == a[i + 1]) {
                    ans.add(a[i]);
                    i--;
                }
            }
            if (ans.size() < 2) {
                out.ansln(0);
            } else {
                out.ansln(ans.get(0) * ans.get(1));
            }
        }

    }

    static class QuickSort {
        private QuickSort() {
        }

        private static void med(long[] a, int low, int x, int y, int z) {
            if (a[z] < a[x]) {
                ArrayUtil.swap(a, low, x);
            } else if (a[y] < a[z]) {
                ArrayUtil.swap(a, low, y);
            } else {
                ArrayUtil.swap(a, low, z);
            }
        }

        static int step(long[] a, int low, int high) {
            int x = low + 1, y = low + (high - low) / 2, z = high - 1;
            if (a[x] < a[y]) {
                med(a, low, x, y, z);
            } else {
                med(a, low, y, x, z);
            }

            int lb = low + 1, ub = high;
            while (true) {
                while (a[lb] < a[low]) {
                    lb++;
                }
                ub--;
                while (a[low] < a[ub]) {
                    ub--;
                }
                if (lb >= ub) {
                    return lb;
                }
                ArrayUtil.swap(a, lb, ub);
                lb++;
            }
        }

    }

    static class HeapSort {
        private HeapSort() {
        }

        private static void heapfy(long[] a, int low, int high, int i, long val) {
            int child = 2 * i - low + 1;
            while (child < high) {
                if (child + 1 < high && a[child] < a[child + 1]) {
                    child++;
                }
                if (val >= a[child]) {
                    break;
                }
                a[i] = a[child];
                i = child;
                child = 2 * i - low + 1;
            }
            a[i] = val;
        }

        static void sort(long[] a, int low, int high) {
            for (int p = (high + low) / 2 - 1; p >= low; p--) {
                heapfy(a, low, high, p, a[p]);
            }
            while (high > low) {
                high--;
                long pval = a[high];
                a[high] = a[low];
                heapfy(a, low, high, low, pval);
            }
        }

    }

    static interface Verified {
    }

    static final class BitMath {
        private BitMath() {
        }

        public static int count(int v) {
            v = (v & 0x55555555) + ((v >> 1) & 0x55555555);
            v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
            v = (v & 0x0f0f0f0f) + ((v >> 4) & 0x0f0f0f0f);
            v = (v & 0x00ff00ff) + ((v >> 8) & 0x00ff00ff);
            v = (v & 0x0000ffff) + ((v >> 16) & 0x0000ffff);
            return v;
        }

        public static int msb(int v) {
            if (v == 0) {
                throw new IllegalArgumentException("Bit not found");
            }
            v |= (v >> 1);
            v |= (v >> 2);
            v |= (v >> 4);
            v |= (v >> 8);
            v |= (v >> 16);
            return count(v) - 1;
        }

    }

    static class LightWriter implements AutoCloseable {
        private final Writer out;
        private boolean autoflush = false;
        private boolean breaked = true;

        public LightWriter(Writer out) {
            this.out = out;
        }

        public LightWriter(OutputStream out) {
            this(new BufferedWriter(new OutputStreamWriter(out, Charset.defaultCharset())));
        }

        public LightWriter print(char c) {
            try {
                out.write(c);
                breaked = false;
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
            return this;
        }

        public LightWriter print(String s) {
            try {
                out.write(s, 0, s.length());
                breaked = false;
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
            return this;
        }

        public LightWriter ans(String s) {
            if (!breaked) {
                print(' ');
            }
            return print(s);
        }

        public LightWriter ans(long l) {
            return ans(Long.toString(l));
        }

        public LightWriter ans(int i) {
            return ans(Integer.toString(i));
        }

        public LightWriter ansln(int... n) {
            for (int n1 : n) {
                ans(n1).ln();
            }
            return this;
        }

        public LightWriter ansln(long... n) {
            for (long n1 : n) {
                ans(n1).ln();
            }
            return this;
        }

        public LightWriter ln() {
            print(System.lineSeparator());
            breaked = true;
            if (autoflush) {
                try {
                    out.flush();
                } catch (IOException ex) {
                    throw new UncheckedIOException(ex);
                }
            }
            return this;
        }

        public void close() {
            try {
                out.close();
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
        }

    }

    static class LightScanner {
        private BufferedReader reader = null;
        private StringTokenizer tokenizer = null;

        public LightScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
        }

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

        public int ints() {
            return Integer.parseInt(string());
        }

        public long longs() {
            return Long.parseLong(string());
        }

        public long[] longs(int length) {
            return IntStream.range(0, length).mapToLong(x -> longs()).toArray();
        }

    }

    static class IntroSort {
        private static int INSERTIONSORT_THRESHOLD = 16;

        private IntroSort() {
        }

        static void sort(long[] a, int low, int high, int maxDepth) {
            while (high - low > INSERTIONSORT_THRESHOLD) {
                if (maxDepth-- == 0) {
                    HeapSort.sort(a, low, high);
                    return;
                }
                int cut = QuickSort.step(a, low, high);
                sort(a, cut, high, maxDepth);
                high = cut;
            }
            InsertionSort.sort(a, low, high);
        }

        public static void sort(long[] a) {
            if (a.length <= INSERTIONSORT_THRESHOLD) {
                InsertionSort.sort(a, 0, a.length);
            } else {
                sort(a, 0, a.length, 2 * BitMath.msb(a.length));
            }
        }

    }

    static final class ArrayUtil {
        private ArrayUtil() {
        }

        public static void swap(long[] a, int x, int y) {
            long t = a[x];
            a[x] = a[y];
            a[y] = t;
        }

    }

    static class InsertionSort {
        private InsertionSort() {
        }

        static void sort(long[] a, int low, int high) {
            for (int i = low; i < high; i++) {
                for (int j = i; j > low && a[j - 1] > a[j]; j--) {
                    ArrayUtil.swap(a, j - 1, j);
                }
            }
        }

    }

    static class Debug {
        private static final String DEBUG_CALL_PATTERN = "^.+\\.debug\\((.+)\\);.*$";
        private static Pattern debugRegex;
        private static boolean enabled = false;
        private static String src;

        public static void enable(String s) {
            enabled = true;
            src = s;
            if (debugRegex == null) {
                debugRegex = Pattern.compile(DEBUG_CALL_PATTERN);
            }
        }

    }
}



Measured memory_bytes: 44848