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.
import sys
input = sys.stdin.readline 

n = int(input()) 
l = [] 
for i in range(n):
    l.append(list(map(int, input().split()))) 
ed = [] 
deg1 = [] 
for i in range(n):
    if(l[i][0] == 1):
        deg1.append(i) 
while(deg1):
    i = deg1.pop(0) 
    if(l[i][0] == 0):
        continue 
    ed.append([i, l[i][1]]) 
    l[l[i][1]][0] -= 1 
    l[l[i][1]][1] ^= i 
    l[i][0] -= 1 
    if(l[l[i][1]][0] == 1):
        deg1.append(l[i][1]) 
print(len(ed))
for i in ed:
    print(*i)