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E. Candies Game
time limit per test
2 seconds
memory limit per test
256 megabytes
standard input
standard output

Iahub is playing an uncommon game. Initially, he has n boxes, numbered 1, 2, 3, ..., n. Each box has some number of candies in it, described by a sequence a 1, a 2, ..., a n. The number a k represents the number of candies in box k.

The goal of the game is to move all candies into exactly two boxes. The rest of n - 2 boxes must contain zero candies. Iahub is allowed to do several (possible zero) moves. At each move he chooses two different boxes i and j, such that a i ≤ a j. Then, Iahub moves from box j to box i exactly a i candies. Obviously, when two boxes have equal number of candies, box number j becomes empty.

Your task is to give him a set of moves such as Iahub to archive the goal of the game. If Iahub can't win the game for the given configuration of boxes, output -1. Please note that in case there exist a solution, you don't need to print the solution using minimal number of moves.


The first line of the input contains integer n (3 ≤ n ≤ 1000). The next line contains n non-negative integers: a 1, a 2, ..., a n — sequence elements. It is guaranteed that sum of all numbers in sequence a is up to 106.


In case there exists no solution, output -1. Otherwise, in the first line output integer c (0 ≤ c ≤ 106), representing number of moves in your solution. Each of the next c lines should contain two integers i and j (1 ≤ i, j ≤ n, i ≠ j): integers i, j in the kth line mean that at the k-th move you will move candies from the j-th box to the i-th one.

3 6 9
2 3
1 3
0 1 0
0 1 1 0

For the first sample, after the first move the boxes will contain 3, 12 and 3 candies. After the second move, the boxes will contain 6, 12 and 0 candies. Now all candies are in exactly 2 boxes.

For the second sample, you can observe that the given configuration is not valid, as all candies are in a single box and they should be in two boxes. Also, any move won't change the configuration, so there exists no solution.

For the third sample, all candies are already in 2 boxes. Hence, no move is needed.