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 . The first line contains an integer n (1 ≤ n ≤ 10000) . The second line contains n integers: x 1 to x n (0 ≤ x i ≤ 10 9 ) . The third line contains n integers: y 1 to y n (0 ≤ y i ≤ 10 9 ) . 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 x i ^= x j . 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 10 6 operations. Assignment a ^= b denotes assignment a = a ^ b , where operation " ^ " is bitwise XOR of two integers.