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 a₁, a₂, ..., 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: a₁, a₂, ..., 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.