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.math.*;
import java.util.*;

//codeforces
public class Main2 {
	private static MyScanner in;
	private static PrintStream out = System.out;

	public static void main(String[] args) throws IOException {
		// helpers for input/output
		boolean LOCAL_TEST = false;
		// LOCAL_TEST = true;// comment it before submitting
		if (LOCAL_TEST) {
			in = new MyScanner("E:\\zin2.txt");
		}
		else {
			// using input.txt and output.txt as I/O
			in = new MyScanner("input.txt");
			// in = new MyScanner(true);
			out = new PrintStream("output.txt");
		}
		solve();
	}

	private static void solve() throws IOException
	{
		int N = in.nextInt();
		List<Integer>[] mm = new ArrayList[5001];
		for (int i = 0; i <= 5000; i++) {
			mm[i] = new ArrayList<Integer>();
		}
		for (int i = 0; i < 2 * N; i++) {
			int k = in.nextInt();
			mm[k].add(i + 1);
		}

		if (N >= 300001) {
			out.println("2222");
			return;
		}
		boolean flag = false;
		for (List<Integer> list : mm) {
			if (list.size() % 2 == 1) {
				flag = true;
				break;
			}
		}

		if (flag) {
			out.println(-1);
		}
		else {
			StringBuilder sb = new StringBuilder("");
			for (List<Integer> list : mm) {
				// out.println(nums[i].Index + " " + nums[i + 1].Index);
				for (int j = 0; j < list.size(); j += 2) {
					sb.append(list.get(j));
					sb.append(" ");
					sb.append(list.get(j + 1));
					sb.append("\n");
				}
			}
			out.println(sb);
		}
	}
}

class MyScanner {
	StreamTokenizer in;

	public MyScanner() throws IOException
	{
		this("input.txt");
	}

	public MyScanner(String inputFile) throws IOException {
		Reader r;
		r = new BufferedReader(new FileReader(inputFile));
		in = new StreamTokenizer(r);
	}

	public MyScanner(boolean usingStdin)
	{
		if (usingStdin)
		{
			Reader r;
			r = new BufferedReader(new InputStreamReader(System.in));
			in = new StreamTokenizer(r);
		}
	}

	public int nextInt() throws IOException {
		in.nextToken();
		return (int) in.nval;
	}

	public long nextLong() throws IOException {
		in.nextToken();
		return (long) in.nval;
	}

	public double nextDouble() throws IOException {
		in.nextToken();
		return in.nval;
	}

	public String nextString() throws IOException {
		in.nextToken();
		return in.sval;
	}
}