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())
d = []
v = []
for i in range(n):
    (a, b) = map(int, input().split())
    v.append([a, b, i])

ans = []
flag = True
while flag:

    flag = False
    for i in sorted(v):
        if v[i[2]][0] == 1:
            a = v[i[2]][2]
            b = v[i[2]][1]
            v[a][0] -= 1
            v[b][0] -= 1
            v[b][1] ^= a
            ans.append((a, b))
            flag = True

print(len(ans))
for i in ans:
    print(*i)

'''
9
2 5
4 3
7 6
8 3
5 0
6 1
3 9
1 0
9 0
'''