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*<sub class="lower-index">*v*</sub> and *s*<sub class="lower-index">*v*</sub>, 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*<sub class="lower-index">*v*</sub> and *s*<sub class="lower-index">*v*</sub> 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* ≤ 2<sup class="upper-index">16</sup>), the number of vertices in the graph.

The *i*-th of the next lines contains numbers *degree*<sub class="lower-index">*i*</sub> and *s*<sub class="lower-index">*i*</sub> (0 ≤ *degree*<sub class="lower-index">*i*</sub> ≤ *n* - 1, 0 ≤ *s*<sub class="lower-index">*i*</sub> &lt; 2<sup class="upper-index">16</sup>), separated by a space.
from collections import deque
n=int(input())
d=[[0,0] for _ in range(n)]
for i in range(n):
    a,b=map(int,input().split())
    d[i][0]=a
    d[i][1]=b
q=deque()
res=[]
for i in range(n):
    if d[i][0]==1:
        q.append(i)
while q:
    node=q.popleft()
    if d[node][0]==1:
        res.append((node,d[node][1]))
        d[node][0]-=1
        d[d[node][1]][0]-=1
        d[d[node][1]][1]^=node
        if d[d[node][0]]==1:
            q.append(d[node][1])
print(len(res))
for a,b in res:
    print(a,b)