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.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Scanner;
import java.util.StringTokenizer;


public class CF199A {
	public static void main(String[] args) throws Exception{
		FastScanner sc = new FastScanner(new File("input.txt"));
		PrintWriter out = new PrintWriter(new File("output.txt"));
		int N =sc.nextInt()*2;
		thing[] array = new thing[N];
		for(int a=0;a<N;a++)array[a]=new thing(sc.nextInt(),a+1);
		Arrays.sort(array);
		StringBuilder SB = new StringBuilder();
		for(int a=0;a<N;a+=2){
			if(array[a].n!=array[a+1].n){
				out.println("-1");
				out.close();
				System.exit(0);
			}
		}
		for(int a=0;a<N;a+=2){
			SB.append(array[a].pos+" "+array[a+1].pos+"\n");
			
		}
		out.print(SB);
		out.close();
		
	}

}
class FastScanner {
	BufferedReader br;
	StringTokenizer st;
	
	public FastScanner(File in) throws Exception{
		br = new BufferedReader(new FileReader(in));
		st = new StringTokenizer(br.readLine().trim());
	}
	public int numTokens() throws Exception {
		if(!st.hasMoreTokens()){
			st = new StringTokenizer(br.readLine().trim());
			return numTokens();
		}
		return st.countTokens();
	}
	public String next() throws Exception {
		if(!st.hasMoreTokens()){
			st = new StringTokenizer(br.readLine().trim());
			return next();
		}
		return st.nextToken();
	}
	public double nextDouble() throws Exception{
		return Double.parseDouble(next());
	}
	public float nextFloat() throws Exception{
		return Float.parseFloat(next());
	}
	public long nextLong() throws Exception{
		return Long.parseLong(next());
	}
	public int nextInt() throws Exception{
		return Integer.parseInt(next());
	}
	public String nextLine() throws Exception{
		return br.readLine().trim();
	}
}
class thing implements Comparable<thing>{
	int n;
	int pos;
	thing(int a, int b){
		this.n=a;
		this.pos=b;
	}
	@Override
	public int compareTo(thing o) {
		return this.n-o.n;
	}
}