CF254A.Cards with Numbers

Petya has got 2_n_ 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 2_n_. 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.

佩佳有 $2n$ 张卡片，每张卡片上写有一个整数。卡片上的数字可以相同。我们用从 $1$ 到 $2n$ 的连续整数对所有卡片进行编号。记编号为 $i$ 的卡片上的数字为 $a_i$。为了和朋友们玩一个有趣的 game，佩佳需要将这些卡片两两配对，使得每一对中的两张卡片上的数字相等。请帮助佩佳完成这一配对。

The first line contains integer n (1 ≤ n ≤ 3·105). The second line contains the sequence of 2_n_ positive integers _a_1, _a_2, ..., a_2_n (1 ≤ a__i ≤ 5000) — the numbers that are written on the cards. The numbers on the line are separated by single spaces.

第一行包含一个整数 $n$（$1 \leq n \leq 3\cdot10^5$）。第二行包含长度为 $2n$ 的正整数序列 $a_1,\,a_2,\,\dots,\,a_{2n}$（$1 \leq a_i \leq 5000$）——即写在卡片上的数字。同一行中的数字以单个空格分隔。

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.

如果无法将卡片分成若干对，使得每对卡片上的数字相同，则在单独一行输出整数 $-1$。但如果存在满足要求的划分方式，则输出 $n$ 对整数，每行一对——即组成每对的卡片的下标。

每行中的两个数字用空格分隔。你可以以任意顺序输出各对，以及每对中两个数字的顺序。若存在多种解法，输出任意一种即可。