Cards with Numbers

题意翻译

题目翻译：有 $2n$ 张上面写了数字的卡片，编号为 $1 ,2,... ,2n$ 。如果两张卡片上的数字相同可凑成一对，问你能否找到一种方法使所有卡片都凑成对？ 输入格式：第一行一个数 $n$ ，第二行 $2n$ 个数，表示卡牌上写的数字。 输出格式：如果能满足要求，则输出共 $n$ 行，每行两个数 $a$ 和 $b$ 。表示编号为 $a$ 的卡片与编号为 $b$ 的卡片应凑成一对。 如果不能，输出 $-1$ 。

题目描述

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 $ a_{i} $ . 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.

输入输出格式

输入格式

The first line contains integer $ n $ ( $ 1<=n<=3·10^{5} $ ). The second line contains the sequence of $ 2n $ positive integers $ a_{1},a_{2},...,a_{2n} $ ( $ 1<=a_{i}<=5000 $ ) — the numbers that are written on the cards. The numbers on the line are separated by single spaces.

输出格式

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.

输入输出样例

输入样例 #1

3
20 30 10 30 20 10

输出样例 #1

4 2
1 5
6 3

输入样例 #2

1
1 2

输出样例 #2

-1