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.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Iterator;



public class P254A
{
	
	/**
	 * @param args
	 * @throws Exception 
	 */
	public static void main(String[] args) throws Exception
	{
		// TODO Auto-generated method stub
//		Scanner in = new Scanner(new FileReader("input.txt")) ;
		InputStream input = new  FileInputStream("input.txt");
		FS in = new FS(input);
		int n = in.nextInt();
		int[] array = new int[5001];
		HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
		n *= 2;
		for (int c=0; c<n; c++)
		{
			int number = in.nextInt();
			if (array[number] == 0) array[number] = c+1;
			else {
				map.put(array[number], c+1);
				array[number] = 0;
			}
			 
		}
		PrintWriter out = new PrintWriter(new File("output.txt"));
		if (map.size() != n/2) out.println(-1);
		else
		{
			Iterator<Integer> it = map.keySet().iterator();
			for (int i=0; i<n/2; i++)
			{
				int key = it.next();
				out.println(key + " " + map.get(key));
			}
		}
		out.close();
	}
	static class FS
	{
	   final private int BUFFER_SIZE = 1 << 17;

	   private DataInputStream din;
	   private byte[] buffer;
	   private int bufferPointer, bytesRead;

	   public FS(InputStream in)
	   {
	      din = new DataInputStream(in);
	      buffer = new byte[BUFFER_SIZE];
	      bufferPointer = bytesRead = 0;
	   }
	   public String nextString() throws Exception
	   {
	       StringBuffer sb=new StringBuffer("");
	       byte c = read();
	       while (c <= ' ') c = read();
	       do
	       {
	           sb.append((char)c);
	           c=read();
	       }while(c>' ');
	       return sb.toString();
	   }
	   public char nextChar() throws Exception
	   {
	       byte c=read();
	       while(c<=' ') c= read();
	       return (char)c;
	   }
	   public int nextInt() throws Exception
	   {
	      int ret = 0;
	      byte c = read();
	      while (c <= ' ') c = read();
	      boolean neg = c == '-';
	      if (neg) c = read();
	      do
	      {
	          ret = ret * 10 + c - '0';
	         c = read();
	      } while (c > ' ');
	      if (neg) return -ret;
	      return ret;
	   }
	   public long nextLong() throws Exception
	   {
	      long ret = 0;
	      byte c = read();
	      while (c <= ' ') c = read();
	      boolean neg = c == '-';
	      if (neg) c = read();
	      do
	      {
	          ret = ret * 10 + c - '0';
	         c = read();
	      } while (c > ' ');
	      if (neg) return -ret;
	      return ret;
	   }
	   private void fillBuffer() throws Exception
	   {
	      bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
	      if (bytesRead == -1) buffer[0] = -1;
	   }

	   private byte read() throws Exception
	   {
	      if (bufferPointer == bytesRead) fillBuffer();
	      return buffer[bufferPointer++];
	   }
	}   
}