There are some tasks which have the following structure: you are given a model, and you can do some operations, you should use these operations to achive the goal. One way to create a new task is to use the same model and same operations, but change the goal. Let's have a try. I have created the following task for Topcoder SRM 557 Div1-Hard: you are given n integers x 1, x 2, ..., x n . You are allowed to perform the assignments (as many as you want) of the following form x i ^= x j (in the original task i and j must be different, but in this task we allow i to equal j ). The goal is to maximize the sum of all x i . Now we just change the goal. You are also given n integers y 1, y 2, ..., y n . You should make x 1, x 2, ..., x n exactly equal to y 1, y 2, ..., y n . In other words, for each i number x i should be equal to y i .

输入描述:

The first line contains an integer n(1 ≤ n ≤ 10000). The second line contains n integers: x1 to xn(0 ≤ xi ≤ 109). The third line contains n integers: y1 to yn(0 ≤ yi ≤ 109).

输出描述:

If there is no solution, output -1.If there is a solution, then in the first line output an integer m(0 ≤ m ≤ 1000000) – the number of assignments you need to perform. Then print m lines, each line should contain two integers i and j(1 ≤ i, j ≤ n), which denote assignment xi ^= xj.If there are multiple solutions you can print any of them. We can prove that under these constraints if there exists a solution then there always exists a solution with no more than 106 operations.

备注:

Assignment a ^= b denotes assignment a = a ^ b, where operation "^" is bitwise XOR of two integers.