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())
ss = [0] * n
degs = [0] * n
leaves = []
for i in range(n):
    degs[i], ss[i] = list(map(int, input().split()))
    if degs[i] == 1:
        leaves.append(i)

edges = []
while len(leaves) != 0:
    leaf = leaves.pop()
    if degs[leaf] == 0:
        continue
    v = ss[leaf]
    edges.append((leaf, v))
    v_deg = degs[v]

    ss[v] = ss[v] ^ leaf
    degs[v] -= 1
    if degs[v] == 1:
        leaves.append(v)

print(len(edges))
for edge in edges:
    print(edge[0], edge[1])




