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.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.StringTokenizer;

public class A {

    public static void main(String[] args) throws FileNotFoundException {
        Scanner in = new Scanner();
        //PrintWriter out = new PrintWriter(System.out);
        PrintWriter out = new PrintWriter(new FileOutputStream(new File("output.txt")));
        int n = in.nextInt();
        int[] data = new int[5001];
        Arrays.fill(data, -1);
        int count = 0;
        ArrayList<Point> list = new ArrayList();
        for (int i = 0; i < 2 * n; i++) {
            int val = in.nextInt();
            if (data[val] == -1) {
                data[val] = i;
                count++;
            } else {
                list.add(new Point(data[val], i));
                count--;
                data[val] = -1;
            }
        }
        if (count != 0) {
            out.println(-1);
        } else {
            for (Point p : list) {
                out.println((p.x + 1) + " " + (p.y + 1));
            }
        }

        out.close();
    }

    static class Dragon implements Comparable<Dragon> {

        int x, y;

        public Dragon(int x, int y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public int compareTo(Dragon o) {
            if (x != o.x) {
                return x - o.x;
            } else {
                return o.y - y;
            }
        }
    }

    static double dist(Point a, Point b) {
        long total = (a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y);
        return Math.sqrt(total);
    }

    static class Point {

        int x, y;

        public Point(int x, int y) {
            this.x = x;
            this.y = y;

        }
    }

    public static boolean nextPer(int[] data) {
        int i = data.length - 1;
        while (i > 0 && data[i] < data[i - 1]) {
            i--;
        }
        if (i == 0) {
            return false;
        }
        int j = data.length - 1;
        while (data[j] < data[i - 1]) {
            j--;
        }
        int temp = data[i - 1];
        data[i - 1] = data[j];
        data[j] = temp;
        Arrays.sort(data, i, data.length);
        return true;
    }

    static class FT {

        int[] data;

        FT(int n) {
            data = new int[n];
        }

        void update(int index, int val) {
            // System.out.println("UPDATE INDEX " + index);
            while (index < data.length) {
                data[index] += val;
                index += index & (-index);

                //    System.out.println("NEXT " +index);
            }
        }

        int get(int index) {
            //  System.out.println("GET INDEX " + index);
            int result = 0;
            while (index > 0) {
                result += data[index];
                index -= index & (-index);
                // System.out.println("BACK " + index);
            }
            return result;
        }
    }

    static int gcd(int a, int b) {
        if (b == 0) {
            return a;
        } else {
            return gcd(b, a % b);
        }
    }

    static int pow(int a, int b) {
        if (b == 0) {
            return 1;
        }
        if (b == 1) {
            return a;
        }
        int val = pow(a, b / 2);
        if (b % 2 == 0) {

            return val * val;
        } else {
            return val * val * a;
        }
    }

//    static Point intersect(Point a, Point b, Point c) {
//        double D = cross(a, b);
//        if (D != 0) {
//            return new Point(cross(c, b) / D, cross(a, c) / D);
//        }
//        return null;
//    }
//
//    static Point convert(Point a, double angle) {
//        double x = a.x * cos(angle) - a.y * sin(angle);
//        double y = a.x * sin(angle) + a.y * cos(angle);
//        return new Point(x, y);
//    }
//    static Point minus(Point a, Point b) {
//        return new Point(a.x - b.x, a.y - b.y);
//    }
//
//    static Point add(Point a, Point b) {
//        return new Point(a.x + b.x, a.y + b.y);
//    }
//
//    static double cross(Point a, Point b) {
//        return a.x * b.y - a.y * b.x;
//
//
//    }
//
//    static class Point {
//
//        int x, y;
//
//        Point(int x, int y) {
//            this.x = x;
//            this.y = y;
//        }
//
//        @Override
//        public String toString() {
//            return "Point: " + x + " " + y;
//        }
//    }
    static class Scanner {

        BufferedReader br;
        StringTokenizer st;

        public Scanner() throws FileNotFoundException {
            //System.setOut(new PrintStream(new BufferedOutputStream(System.out), true));
            //br = new BufferedReader(new InputStreamReader(System.in));
            br = new BufferedReader(new FileReader(new File("input.txt")));
        }

        public String next() {


            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    throw new RuntimeException();
                }
            }
            return st.nextToken();
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public String nextLine() {
            st = null;
            try {
                return br.readLine();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }

        public boolean endLine() {
            try {
                String next = br.readLine();
                while (next != null && next.trim().isEmpty()) {
                    next = br.readLine();
                }
                if (next == null) {
                    return true;
                }
                st = new StringTokenizer(next);
                return st.hasMoreTokens();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }
    }
}