B'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 xe2 x80 x89- xe2 x80 x891 he wrote down two integers, degreev and sv, 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 degreev and sv 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. The first line contains integer n (1 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89n xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89216), the number of vertices in the graph. The i-th of the next lines contains numbers degreei and si (0 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89degreei xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89n xe2 x80 x89- xe2 x80 x891, 0 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89si xe2 x80 x89< xe2 x80 x89216), separated by a space. 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 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89a xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89n xe2 x80 x89- xe2 x80 x891, 0 xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89b xe2 x80 x89 xe2 x89 xa4 xe2 x80 x89n xe2 x80 x89- xe2 x80 x891), corresponding to edge (a, xe2 x80 x89b). Edges can be printed in any order; vertices of the edge can also be printed in any order. 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".'...