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.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Inet4Address;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Hashtable;
import java.util.Set;
import java.util.StringTokenizer;

public class Solve {

	void sieveOfEratosthenes(int n) {
		boolean prime[] = new boolean[n + 1];
		for (int i = 0; i < n; i++)
			prime[i] = true;

		for (int p = 2; p * p <= n; p++) {
			if (prime[p] == true) {
				for (int i = p * p; i <= n; i += p)
					prime[i] = false;
			}
		}
		for (int i = 2; i <= n; i++) {
			if (prime[i] == true)
				System.out.print(i + " ");
		}
	}

	/**
	 * @param args
	 * @throws NumberFormatException
	 * @throws IOException
	 */
	public static void main(String[] args) throws NumberFormatException, IOException {

//		RizkScanner sc = new RizkScanner();
//		PrintWriter out = new PrintWriter(System.out);
		RizkScanner sc = new RizkScanner("input.txt");
		PrintWriter out = new PrintWriter("output.txt");
		int n = sc.nextInt();
		int[] arr = new int[5001];
		Hashtable<Integer, ArrayList<Integer>> index = new Hashtable<Integer, ArrayList<Integer>>();
		for (int i = 0; i < 2 * n; i++) {
			int x = sc.nextInt();
			if (!index.containsKey(x)) {
				ArrayList<Integer> z = new ArrayList<Integer>();
				z.add(i + 1);
				index.put(x, z);
			} else
				index.get(x).add(i + 1);
			arr[x]++;
		}
		for (int i = 0; i < 5001; i++) {
			if (arr[i] % 2 != 0) {
				out.print(-1);
				out.flush();
				return;
			}
		}
		for (int i = 0; i < 5001; i++) {
			if (arr[i] != 0) {
				for (int j = 0; j < index.get(i).size(); j++) {
					out.println(index.get(i).get(j) + " " + index.get(i).get(j + 1));
					j++;
				}
			}
		}
		out.flush();
	}

	static class RizkScanner {
		BufferedReader br;
		StringTokenizer st;

		RizkScanner(String file) throws IOException {
			br = new BufferedReader(new FileReader(new File(file)));
		}

		RizkScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() throws IOException {
			while (st == null || !st.hasMoreTokens()) {
				st = new StringTokenizer(br.readLine());
			}
			return st.nextToken();
		}

		String nextLine() throws IOException {
			return br.readLine();
		}

		int nextInt() throws NumberFormatException, IOException {
			return Integer.parseInt(next());
		}

		long nextLong() throws NumberFormatException, IOException {
			return Long.parseLong(next());
		}

		double nextDouble() throws NumberFormatException, IOException {
			return Double.parseDouble(next());
		}

		float nextFloat() throws NumberFormatException, IOException {
			return Float.parseFloat(next());
		}
	}

}