Problem: Petya has got 2n cards, each card contains some integer. The numbers on the cards can be the same. Let's index all cards by consecutive integers from 1 to 2n. We'll denote the number that is written on a card with number i, as ai. In order to play one entertaining game with his friends, Petya needs to split the cards into pairs so that each pair had equal numbers on the cards. Help Petya do that.

Input

The first line contains integer n (1 ≤ n ≤ 3·105). The second line contains the sequence of 2n positive integers a1, a2, ..., a2n (1 ≤ ai ≤ 5000) — the numbers that are written on the cards. The numbers on the line are separated by single spaces.

Output

If it is impossible to divide the cards into pairs so that cards in each pair had the same numbers, print on a single line integer -1. But if the required partition exists, then print n pairs of integers, a pair per line — the indices of the cards that form the pairs.

Separate the numbers on the lines by spaces. You can print the pairs and the numbers in the pairs in any order. If there are multiple solutions, print any of them.

Examples

Input

3
20 30 10 30 20 10


Output

4 2
1 5
6 3


Input

1
1 2


Output

-1

Solution (in JAVA):

Solution:
import java.io.*;
import java.io.OutputStream;
import java.io.IOException;
import java.util.StringTokenizer;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Omar-Handouk
 */
public class Main {
    public static void main(String[] args) throws IOException{
        /*InputStream inputStream = System.in;
        OutputStream outputStream = System.out;*/
        Scanner in = new Scanner(new FileInputStream(new File("input.txt")));
        OutputWriter out = new OutputWriter(new FileOutputStream("output.txt"));
        Cards_with_Numbers solver = new Cards_with_Numbers();
        solver.solve(1, in, out);
        out.close();
    }

    static class Cards_with_Numbers {
        public void solve(int testNumber, Scanner in, OutputWriter out) {

            int[] pos = new int[5001];

            int n = in.nextInt();
            int left = 2 * n;

            StringBuilder ans = new StringBuilder();
            for (int i = 1; i <= 2 * n; ++i) {
                int num = in.nextInt();
                if (pos[num] == 0)
                    pos[num] = i;
                else {
                    ans.append(pos[num] + " " + i + "\n");
                    left -= 2;
                    pos[num] = 0;
                }
            }

            if (left != 0)
                out.println(-1);
            else
                out.print(ans);
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void close() {
            writer.close();
        }

        public void println(int i) {
            writer.println(i);
        }

    }

    static class Scanner {
        private BufferedReader bufferedReader;
        private StringTokenizer stringTokenizer;

        public Scanner(InputStream stream) {
            bufferedReader = new BufferedReader(new InputStreamReader(stream));
        }

        public String next() {
            while (stringTokenizer == null || !stringTokenizer.hasMoreTokens()) {
                try {
                    stringTokenizer = new StringTokenizer(bufferedReader.readLine());
                } catch (IOException Error) {
                    Error.printStackTrace();
                    ;
                }
            }

            return stringTokenizer.nextToken();
        }

        public int nextInt() {
            int number = Integer.parseInt(this.next());

            return number;
        }

    }
}