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.lang.reflect.*;
import java.util.*;

public class A {

	final int MOD = (int)1e9 + 7;
	final double eps = 1e-12;
	final int INF = (int)1e9;
	
	public A () {
		int N = sc.nextInt();
		int [] Z = new int[6000];
		Integer [] A = sc.nextInts();
		
		List<int[]> P = new ArrayList<int[]>();
		
		for (int i = 0; i < 2*N; ++i) {
			int x = A[i];
			if (Z[x] != 0) {
				P.add(new int [] { Z[x], i+1 });
				Z[x] = 0;
			} else
				Z[x] = i+1;
		}
		
		if (P.size() == N) {
			for (int [] p : P)
				print(p);
		} else
			exit(-1);
	}

	////////////////////////////////////////////////////////////////////////////////////
	
	static MyScanner sc;
	
	static class MyScanner {
		public String next() {
			newLine();
			return line[index++];
		}
		
		public char nextChar() {
			return next().charAt(0);
		}
				
		public int nextInt() {
			return Integer.parseInt(next());
		}
		
		public long nextLong() {
			return Long.parseLong(next());
		}
		
		public double nextDouble() {
			return Double.parseDouble(next());
		}
		
		public String nextLine() {
			line = null;
			return readLine();
		}
		
		public String [] nextStrings() {
			line = null;
			return readLine().split(" ");
		}
		
		public char [] nextChars() {
			return next().toCharArray();
		}

		public Integer [] nextInts() {
			String [] L = nextStrings();
			Integer [] res = new Integer [L.length];
			for (int i = 0; i < L.length; ++i)
				res[i] = Integer.parseInt(L[i]);
			return res;
		}	
		
		public Long [] nextLongs() {
			String [] L = nextStrings();
			Long [] res = new Long [L.length];
			for (int i = 0; i < L.length; ++i)
				res[i] = Long.parseLong(L[i]);
			return res;
		}

		public Double [] nextDoubles() {
			String [] L = nextStrings();
			Double [] res = new Double [L.length];
			for (int i = 0; i < L.length; ++i)
				res[i] = Double.parseDouble(L[i]);
			return res;
		}

		//////////////////////////////////////////////
		
		private boolean eol() {
			return index == line.length;
		}

		private String readLine() {
			try {
				return r.readLine();
			} catch (Exception e) {
				throw new Error(e);
			}
		}
		private final BufferedReader r;

		MyScanner () {
			this(new BufferedReader(IO.reader()));
			//this(new BufferedReader(new InputStreamReader(System.in)));
		}
		
		MyScanner(BufferedReader r) {
			try {
				this.r = r;
				while (!r.ready())
					Thread.sleep(1);
				start();
			} catch (Exception e) {
				throw new Error(e);
			}
		}
		
		private String [] line;
		private int index;

		private void newLine() {
			if (line == null || eol()) {
				line = readLine().split(" ");
				index = 0;
			}
		}		
	}
	
	static void print (Object o, Object... a) {
		pw.println(build(o, a));
	}

	static void exit (Object o, Object... a) {
		print(o, a);
		exit();
	}

	static void exit () {
		pw.close();
		System.out.flush();
		System.err.println("------------------");
		System.err.println("Time: " + ((millis() - t) / 1000.0));
		System.exit(0);
	}
	
	void NO() {
		throw new Error("NO!");
	}
	
	////////////////////////////////////////////////////////////////////////////////////
	
	static String build(Object... a) {
		StringBuilder b = new StringBuilder();
		for (Object o : a)
			append(b, o);
		return b.toString().trim();		
	}
	
	static void append(StringBuilder b, Object o) {
		if (o.getClass().isArray()) {
			int L = Array.getLength(o);
			for (int i = 0; i < L; ++i)
				append(b, Array.get(o, i));
		} else if (o instanceof Iterable<?>) {
			for (Object p : (Iterable<?>)o)
				append(b, p);
		} else
			b.append(" ").append(o);		
	}
	
	////////////////////////////////////////////////////////////////////////////////////
	
	public static void main(String[] args) {
		sc = new MyScanner ();
		new A();
		exit();
	}

	static void start() {
		t = millis();
	}

	//static PrintWriter pw = new PrintWriter(System.out);
	static PrintWriter pw = new PrintWriter(IO.writer());
	
	static long t;
	
	static long millis() {
		return System.currentTimeMillis();
	}
	
	static class IO {
		static Reader reader () {
			try {
				return new FileReader("input.txt");
			} catch (Exception e) {
				throw new Error(e);
			}
		}
		
		static Writer writer() { 
			try {
				return new FileWriter("output.txt");
			} catch (Exception e) {
				throw new Error(e);
			}
		}
	}
}