CF501C.Misha and Forest

Let's define a forest as a non-directed acyclic graph (also without loops and parallel edges). One day Misha played with the forest consisting of n vertices. For each vertex v from 0 to n - 1 he wrote down two integers, degree__v and s__v, were the first integer is the number of vertices adjacent to vertex v, and the second integer is the XOR sum of the numbers of vertices adjacent to v (if there were no adjacent vertices, he wrote down 0).

Next day Misha couldn't remember what graph he initially had. Misha has values degree__v and s__v left, though. Help him find the number of edges and the edges of the initial graph. It is guaranteed that there exists a forest that corresponds to the numbers written by Misha.

我们定义森林为一个无向的无环图（且不含自环和平行边）。某天，米沙玩了一个包含 $n$ 个顶点的森林。对于从 $0$ 到 $n-1$ 的每个顶点 $v$，他记录了两个整数：$\text{degree}_v$ 和 $s_v$，其中第一个整数表示与顶点 $v$ 相邻的顶点个数，第二个整数表示与顶点 $v$ 相邻的所有顶点编号的异或和（若没有相邻顶点，则记为 $0$）。

第二天，米沙已记不清他最初画的是哪个图了，但他仍保留着所有 $\text{degree}_v$ 和 $s_v$ 的值。请你帮助他还原出初始图的边数及所有具体的边。题目保证存在一个森林与米沙所记录的数值完全对应。

The first line contains integer n (1 ≤ n ≤ 216), the number of vertices in the graph.

The i-th of the next lines contains numbers degree__i and s__i (0 ≤ degree__i ≤ n - 1, 0 ≤ s__i < 216), separated by a space.

第一行包含一个整数 $n$（$1 \leq n \leq 2^{16}$），表示图中顶点的数量。

接下来的第 $i$ 行包含两个数 $\text{degree}_i$ 和 $s_i$（$0 \leq \text{degree}_i \leq n - 1$，$0 \leq s_i < 2^{16}$），以空格分隔。

In the first line print number m, the number of edges of the graph.

Next print m lines, each containing two distinct numbers, a and b (0 ≤ a ≤ n - 1, 0 ≤ b ≤ n - 1), corresponding to edge (a, b).

Edges can be printed in any order; vertices of the edge can also be printed in any order.

第一行输出整数 $m$，表示图中边的数量。

接下来输出 $m$ 行，每行包含两个不同的整数 $a$ 和 $b$（满足 $0 \le a \le n - 1$，$0 \le b \le n - 1$），对应一条边 $(a, b)$。

边的输出顺序可以任意；每条边的两个顶点的输出顺序也可以任意。

The XOR sum of numbers is the result of bitwise adding numbers modulo 2. This operation exists in many modern programming languages. For example, in languages C++, Java and Python it is represented as "^", and in Pascal — as "xor".

数字的异或和（XOR sum）是将数字按位相加后对 2 取模的结果。该运算存在于许多现代编程语言中。例如，在 C++、Java 和 Python 中用符号 “^” 表示，在 Pascal 中则用 “xor” 表示。