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.*;    //PrintWriter
import java.math.*;  //BigInteger, BigDecimal
import java.util.*;  //StringTokenizer, ArrayList


public class R155_Div2_A
{	
	FastReader in;
	PrintWriter out;
	
	public static void main(String[] args)  {
		new R155_Div2_A().runWithFiles();
	}
	
	void run()
	{		
		in = new FastReader(System.in);
		out = new PrintWriter(System.out);
		solve();
		out.close();
	}
	
	void runWithFiles() {
		//in = new FastReader(new File("src/input.txt"));
		in = new FastReader(new File("input.txt"));
		try {
			//out = new PrintWriter(new File("src/output.txt"));
			out = new PrintWriter(new File("output.txt"));
		} 
		catch (FileNotFoundException e) {
			e.printStackTrace();
		}
		solve();
		out.close();
	}
	
	void solve()
	{
		int n = in.nextInt();
		
		int[] a = new int[2*n + 1];
		for (int i = 1; i <= 2*n; i++) 
			a[i] = in.nextInt();
		
        String[] ans = new String[n];
        int[] ind = new int[3 * 100_000 + 1];
        int cnt = 0;
        for (int i = 1; i <= 2 * n; i++)
            if (ind[a[i]] == 0)
                ind[a[i]] = i;
            else
            {
                ans[cnt++] = i + " " + ind[a[i]];
                ind[a[i]] = 0;
            }

        if (cnt == n)
            for (String st : ans)
                out.println(st);
        else
        	out.println(-1);
	}

	//-----------------------------------------------------

	
	class FastReader
	{
	    BufferedReader br;
	    StringTokenizer tokenizer;
	    
	    public FastReader(InputStream stream)
	    {
	        br = new BufferedReader(new InputStreamReader(stream));
	        tokenizer = null;
	    }
		public FastReader(File f) {
			try {
				br = new BufferedReader(new FileReader(f));
				tokenizer = null;
			}
			catch (FileNotFoundException e) {
				e.printStackTrace();
			}
		}
	    
	    private String next() {
	        while (tokenizer == null || !tokenizer.hasMoreTokens())
	            try {
	            	tokenizer = new StringTokenizer(br.readLine());
	            }
	            catch (IOException e) {
	                throw new RuntimeException(e);
	            }
	        return tokenizer.nextToken();
	    }
		public String nextLine() {
			try	{
				return br.readLine();
			}
			catch(Exception e) {
				throw(new RuntimeException());
			}
		}

	    int nextInt() {
	        return Integer.parseInt(next());
	    }
	    long nextLong() {
	        return Long.parseLong(next());
	    }
	    double nextDouble() {
	        return Double.parseDouble(next());
	    }	    
	    BigInteger nextBigInteger() {
	        return new BigInteger(next());
	    }
	    BigDecimal nextBigDecimal() {
	        return new BigDecimal(next());
	    }
	}
}