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.

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.

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.