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:
/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *cf254a
 * @author zulkan
 */
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import static java.lang.System.*;
import java.math.BigInteger;
import java.util.List;
import java.util.ArrayList;
import java.util.Scanner;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;


import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.List;
import java.util.ArrayList;
import java.util.Scanner;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;

public class cf254a {

    static BufferedReader br;
    static Scanner sc;
    static PrintWriter out;

    public static void initA() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
            //br = new BufferedReader(new FileReader("input.txt"));
            sc = new Scanner(System.in);
            //out = new PrintWriter("output.txt");
            out = new PrintWriter(System.out);
        } catch (Exception e) {
        }
    }

    public static void initB() {
        try {

            br = new BufferedReader(new FileReader("input.txt"));
            sc = new Scanner(new FileReader("input.txt"));
            out = new PrintWriter("output.txt");

        } catch (Exception e) {
        }
    }

    public static String getString() {
        try {
            return br.readLine();
        } catch (Exception e) {
        }
        return "";
    }

    public static Integer getInt() {
        try {
            return Integer.parseInt(br.readLine());
        } catch (Exception e) {
        }
        return 0;
    }

    public static Integer[] getIntArr() {
        try {
            StringTokenizer temp = new StringTokenizer(br.readLine());
            int n = temp.countTokens();
            Integer temp2[] = new Integer[n];
            for (int i = 0; i < n; i++) {
                temp2[i] = Integer.parseInt(temp.nextToken());
            }
            return temp2;
        } catch (Exception e) {
        }
        return null;
    }

    public static int getMax(Integer[] ar) {
        int t = ar[0];
        for (int i = 0; i < ar.length; i++) {
            if (ar[i] > t) {
                t = ar[i];
            }
        }
        return t;
    }

    public static void print(Object a) {
        out.println(a);
    }

    public static int nextInt() {
        return sc.nextInt();
    }

    public static double nextDouble() {
        return sc.nextDouble();
    }

    public static BigInteger getFact(long in) {
        BigInteger ot = new BigInteger("1");
        for (Integer i = 1; i <= in; i++) {
            ot = ot.multiply(new BigInteger(i.toString()));
        }
        return ot;
    }

    public static String gbase(int a, int base) {
        StringBuilder out = new StringBuilder();
        int temp = a;
        while (temp > 0) {
            //out = temp % base + out;
            out.insert(0, temp % base);
            temp /= base;
        }
        return out.toString();
    }

    public static void main(String[] ar) {
        initB();
        int n = getInt() * 2;
        Integer arr[] = getIntArr();

        HashMap<Integer, ArrayList> data = new HashMap<Integer, ArrayList>();

        for (int i = 0; i < n; i++) {
            if (!data.containsKey(arr[i])) {
                //print('x');
                ArrayList temp = new ArrayList();
                temp.add(i + 1);
                data.put(arr[i], temp);
            } else {
                //print('w');
                ArrayList temp = data.get(arr[i]);
                temp.add(i + 1);

                data.put(arr[i], temp);
            }
        }
        String xx = "";

        for (ArrayList temp : data.values()) {
            if (temp.size() % 2 != 0) {
                //print(temp.size() + " : " + temp);
                print(-1);
                out.flush();
                return;
            }
        }
        for (ArrayList temp : data.values()) {
            for (int i = 0; i < temp.size(); i += 2) {
                print(temp.get(i) + " " + temp.get(i + 1));

            }
        }


        out.flush();

    }
}