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 10 6 . In case there exists no solution, output -1. Otherwise, in the first line output integer c (0 ≤ c ≤ 10 6 ) , 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 k th line mean that at the k -th move you will move candies from the j -th box to the i -th one. 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