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.

Input

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 $10^{6}$.

Output

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.

Note

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.