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:
//package CodeForces;

/*In the name of Allah the Most Merciful.
 * Author
 * Md. Toufiqul Islam
 * Dept. Of CSE
 * Ahsanullah University Of Science And Technology
*/
import java.io.*;
import java.math.*;
import java.util.*;
public class R155A {
    public static void main(String[] args) throws FileNotFoundException {
        File file = new File("input.txt");
        InputReader sc = new InputReader(new FileInputStream(file));
        OutputWriter out =  new OutputWriter(new FileOutputStream(("output.txt")));
       /*InputReader sc = new InputReader(System.in);
       OutputWriter out = new OutputWriter(System.out);*/

        int n = sc.nextInt();

       List<Integer> list[] = new ArrayList[5000+1];

       for(int i=0;i<list.length;i++){
           list[i] = new ArrayList<>();
       }


      Map<Integer,Integer> map = new HashMap<>();


        for(int i=0;i<2*n;i++){
            int a = sc.nextInt();

           if(map.containsKey(a)){
               map.put(a,map.get(a)+1);
           }
           else{
               map.put(a,1);
           }
              list[a].add(i+1);


        }

        boolean isAnyOddValue = false;

        Set<Integer> set = map.keySet();

       Iterator<Integer> it = set.iterator();

       while (it.hasNext()){
           if(map.get(it.next())%2!=0){
               isAnyOddValue = true;
               break;
           }
       }

       if(isAnyOddValue){
           out.println("-1");
       }
       else{
           it = set.iterator();

           while (it.hasNext()) {
               int a = it.next();

               if (map.get(a) % 2 == 0) {
                   List<Integer> pos = list[a];
                   for (int i = 0; i < pos.size() - 1; i+=2) {
                       out.println(pos.get(i) + " " + pos.get(i + 1));
                   }
               }
           }
       }

        out.close();
    }

     public static class InputReader {
             private boolean finished = false;

             private InputStream stream;
             private byte[] buf = new byte[1024];
             private int curChar;
             private int numChars;
             private SpaceCharFilter filter;

             public InputReader(InputStream stream) {
                 this.stream = stream;
             }

             public int read() {
                 if (numChars == -1) {
                     throw new InputMismatchException();
                 }
                 if (curChar >= numChars) {
                     curChar = 0;
                     try {
                         numChars = stream.read(buf);
                     } catch (IOException e) {
                         throw new InputMismatchException();
                     }
                     if (numChars <= 0) {
                         return -1;
                     }
                 }
                 return buf[curChar++];
             }

             public int peek() {
                 if (numChars == -1) {
                     return -1;
                 }
                 if (curChar >= numChars) {
                     curChar = 0;
                     try {
                         numChars = stream.read(buf);
                     } catch (IOException e) {
                         return -1;
                     }
                     if (numChars <= 0) {
                         return -1;
                     }
                 }
                 return buf[curChar];
             }

             public int nextInt() {
                 int c = read();
                 while (isSpaceChar(c)) {
                     c = read();
                 }
                 int sgn = 1;
                 if (c == '-') {
                     sgn = -1;
                     c = read();
                 }
                 int res = 0;
                 do {
                     if (c < '0' || c > '9') {
                         throw new InputMismatchException();
                     }
                     res *= 10;
                     res += c - '0';
                     c = read();
                 } while (!isSpaceChar(c));
                 return res * sgn;
             }

             public long nextLong() {
                 int c = read();
                 while (isSpaceChar(c)) {
                     c = read();
                 }
                 int sgn = 1;
                 if (c == '-') {
                     sgn = -1;
                     c = read();
                 }
                 long res = 0;
                 do {
                     if (c < '0' || c > '9') {
                         throw new InputMismatchException();
                     }
                     res *= 10;
                     res += c - '0';
                     c = read();
                 } while (!isSpaceChar(c));
                 return res * sgn;
             }

             public String nextString() {
                 int c = read();
                 while (isSpaceChar(c)) {
                     c = read();
                 }
                 StringBuilder res = new StringBuilder();
                 do {
                     if (Character.isValidCodePoint(c)) {
                         res.appendCodePoint(c);
                     }
                     c = read();
                 } while (!isSpaceChar(c));
                 return res.toString();
             }

             public boolean isSpaceChar(int c) {
                 if (filter != null) {
                     return filter.isSpaceChar(c);
                 }
                 return isWhitespace(c);
             }

             public static boolean isWhitespace(int c) {
                 return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
             }

             private String readLine0() {
                 StringBuilder buf = new StringBuilder();
                 int c = read();
                 while (c != '\n' && c != -1) {
                     if (c != '\r') {
                         buf.appendCodePoint(c);
                     }
                     c = read();
                 }
                 return buf.toString();
             }

             public String readLine() {
                 String s = readLine0();
                 while (s.trim().length() == 0) {
                     s = readLine0();
                 }
                 return s;
             }

             public String readLine(boolean ignoreEmptyLines) {
                 if (ignoreEmptyLines) {
                     return readLine();
                 } else {
                     return readLine0();
                 }
             }

             public BigInteger readBigInteger() {
                 try {
                     return new BigInteger(nextString());
                 } catch (NumberFormatException e) {
                     throw new InputMismatchException();
                 }
             }

             public char nextCharacter() {
                 int c = read();
                 while (isSpaceChar(c)) {
                     c = read();
                 }
                 return (char) c;
             }

             public double nextDouble() {
                 int c = read();
                 while (isSpaceChar(c)) {
                     c = read();
                 }
                 int sgn = 1;
                 if (c == '-') {
                     sgn = -1;
                     c = read();
                 }
                 double res = 0;
                 while (!isSpaceChar(c) && c != '.') {
                     if (c == 'e' || c == 'E') {
                         return res * Math.pow(10, nextInt());
                     }
                     if (c < '0' || c > '9') {
                         throw new InputMismatchException();
                     }
                     res *= 10;
                     res += c - '0';
                     c = read();
                 }
                 if (c == '.') {
                     c = read();
                     double m = 1;
                     while (!isSpaceChar(c)) {
                         if (c == 'e' || c == 'E') {
                             return res * Math.pow(10, nextInt());
                         }
                         if (c < '0' || c > '9') {
                             throw new InputMismatchException();
                         }
                         m /= 10;
                         res += (c - '0') * m;
                         c = read();
                     }
                 }
                 return res * sgn;
             }

             public boolean isExhausted() {
                 int value;
                 while (isSpaceChar(value = peek()) && value != -1) {
                     read();
                 }
                 return value == -1;
             }

             public String next() {
                 return nextString();
             }

             public SpaceCharFilter getFilter() {
                 return filter;
             }

             public void setFilter(SpaceCharFilter filter) {
                 this.filter = filter;
             }

             public interface SpaceCharFilter {
                 public boolean isSpaceChar(int ch);
             }
             public int[] nextIntArray(int n){
                 int[] array=new int[n];
                 for(int i=0;i<n;++i)array[i]=nextInt();
                 return array;
             }
             public int[] nextSortedIntArray(int n){
                 int array[]=nextIntArray(n);
                 Arrays.sort(array);
                 return array;
             }
             public int[] nextSumIntArray(int n){
                 int[] array=new int[n];
                 array[0]=nextInt();
                 for(int i=1;i<n;++i)array[i]=array[i-1]+nextInt();
                 return array;
             }
             public long[] nextLongArray(int n){
                 long[] array=new long[n];
                 for(int i=0;i<n;++i)array[i]=nextLong();
                 return array;
             }
             public long[] nextSumLongArray(int n){
                 long[] array=new long[n];
                 array[0]=nextInt();
                 for(int i=1;i<n;++i)array[i]=array[i-1]+nextInt();
                 return array;
             }
             public long[] nextSortedLongArray(int n){
                 long array[]=nextLongArray(n);
                 Arrays.sort(array);
                 return array;
             }
         }
      public static class OutputWriter {
              private final PrintWriter writer;

              public OutputWriter(OutputStream outputStream) {
                  writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
              }

              public OutputWriter(Writer writer) {
                  this.writer = new PrintWriter(writer);
              }

              public void print(char[] array) {
                  writer.print(array);
              }

              public void print(Object... objects) {
                  for (int i = 0; i < objects.length; i++) {
                      if (i != 0) {
                          writer.print(' ');
                      }
                      writer.print(objects[i]);
                  }
              }

              public void print(int[] array) {
                  for (int i = 0; i < array.length; i++) {
                      if (i != 0) {
                          writer.print(' ');
                      }
                      writer.print(array[i]);
                  }
              }

              public void print(double[] array) {
                  for (int i = 0; i < array.length; i++) {
                      if (i != 0) {
                          writer.print(' ');
                      }
                      writer.print(array[i]);
                  }
              }

              public void print(long[] array) {
                  for (int i = 0; i < array.length; i++) {
                      if (i != 0) {
                          writer.print(' ');
                      }
                      writer.print(array[i]);
                  }
              }

              public void println(int[] array) {
                  print(array);
                  writer.println();
              }

              public void println(double[] array) {
                  print(array);
                  writer.println();
              }

              public void println(long[] array) {
                  print(array);
                  writer.println();
              }

              public void println() {
                  writer.println();
              }

              public void println(Object... objects) {
                  print(objects);
                  writer.println();
              }

              public void print(char i) {
                  writer.print(i);
              }

              public void println(char i) {
                  writer.println(i);
              }

              public void println(char[] array) {
                  writer.println(array);
              }

              public void printf(String format, Object... objects) {
                  writer.printf(format, objects);
              }

              public void close() {
                  writer.close();
              }

              public void flush() {
                  writer.flush();
              }

              public void print(long i) {
                  writer.print(i);
              }

              public void println(long i) {
                  writer.println(i);
              }

              public void print(int i) {
                  writer.print(i);
              }

              public void println(int i) {
                  writer.println(i);
              }

              public void separateLines(int[] array) {
                  for (int i : array) {
                      println(i);
                  }
              }
          }

}