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.
n = int(input())
degrees, mas, masiv = 0, [], []
for i in range(n):
    degree, s = map(int, input().split())
    if degree == 1:
        masiv.append(i)
    degrees += degree
    mas.append([degree, s])
print(degrees // 2)
while masiv:
    last_elem = masiv.pop(-1)
    relax = mas[last_elem][1]
    if mas[last_elem][0] != 1:
        continue
    mas[last_elem][0] -= 1
    mas[last_elem][1] ^= relax
    mas[relax][0] -= 1
    mas[relax][1] ^= last_elem
    print(relax, last_elem)
    if mas[relax][0] == 1:
        masiv.append(relax)
