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, degreev and sv, 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 degreev and sv 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.

Input

The first line contains integer n (1 ≤ n ≤ 216), the number of vertices in the graph.

The i-th of the next lines contains numbers degreei and si (0 ≤ degreei ≤ n - 1, 0 ≤ si < 216), separated by a space.

Output

In the first line print number m, the number of edges of the graph.

Next print m lines, each containing two distinct numbers, a and b (0 ≤ a ≤ n - 1, 0 ≤ b ≤ n - 1), corresponding to edge (a, b).

Edges can be printed in any order; vertices of the edge can also be printed in any order.

Examples

Input

3
2 3
1 0
1 0


Output

2
1 0
2 0


Input

2
1 1
1 0


Output

1
0 1

Note

The XOR sum of numbers is the result of bitwise adding numbers modulo 2. This operation exists in many modern programming languages. For example, in languages C++, Java and Python it is represented as "^", and in Pascal — as "xor".
"""Template for Python Competitive Programmers prepared by Mayank Chaudhary """

# to use the print and division function of Python3
from __future__ import division, print_function

"""value of mod"""
MOD = 998244353
mod = 10**9 + 7

"""use resource"""
# import resource
# resource.setrlimit(resource.RLIMIT_STACK, [0x100000000, resource.RLIM_INFINITY])

"""for factorial"""

# def prepare_factorial():
#     fact = [1]
#     for i in range(1, 100005):
#         fact.append((fact[-1] * i) % mod)
#     ifact = [0] * 100005
#     ifact[100004] = pow(fact[100004], mod - 2, mod)
#     for i in range(100004, 0, -1):
#         ifact[i - 1] = (i * ifact[i]) % mod
#
#     return fact, ifact

"""uncomment next 4 lines while doing recursion based question"""
import threading
threading.stack_size(2**27)
import sys
sys.setrecursionlimit(30000)


"""uncomment modules according to your need"""
# from bisect import bisect_left, bisect_right, insort
# import itertools
# from math import floor, ceil, sqrt, degrees, atan, pi
# from heapq import heappop, heapify, heappush
# from random import randint as rn
# from Queue import Queue as Q
from collections import Counter, defaultdict, deque

'''
def modinv(n, p):
    return pow(n, p - 2, p)
'''


# def ncr(n, r,  fact, ifact):  # for using this uncomment the lines calculating fact and ifact
#     t = ((fact[n]) * (ifact[r] % mod)) % mod
#     return t



def get_ints(): return map(int, sys.stdin.readline().strip().split())
def get_array(): return list(map(int, sys.stdin.readline().strip().split()))
def input(): return sys.stdin.readline().strip()


def GCD(x, y):
    while (y):
        x, y = y, x % y
    return x
#
# def lcm(x, y):
#     return (x*y)//(GCD(x, y))
#
# def get_xor(n):
#     return [n,1,n+1,0][n%4]

"""*******************************************************"""

def main():

    n = int(input())
    Arr = []
    leaf = deque()
    count = 0
    for i in range(n):
        deg, xor = get_ints()
        Arr.append([deg, xor])
        count += deg
        if deg==1:
            leaf.append(i)

    print(count//2)
    # print(leaf)

    while leaf:
        son = leaf.popleft()
        if Arr[son][0]==0:       # degree khtm hogye
            continue
        Arr[son][0] -= 1
        father = Arr[son][1]
        Arr[father][0] -= 1
        Arr[father][1] ^= son

        if Arr[father][0]==1:
            leaf.append(father)

        print(son, father)










""" -------- Python 2 and 3 footer by Pajenegod and c1729 ---------"""

py2 = round(0.5)
if py2:
    from future_builtins import ascii, filter, hex, map, oct, zip

    range = xrange

import os, sys
from io import IOBase, BytesIO

BUFSIZE = 8192


class FastIO(BytesIO):
    newlines = 0

    def __init__(self, file):
        self._file = file
        self._fd = file.fileno()
        self.writable = "x" in file.mode or "w" in file.mode
        self.write = super(FastIO, self).write if self.writable else None

    def _fill(self):
        s = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
        self.seek((self.tell(), self.seek(0, 2), super(FastIO, self).write(s))[0])
        return s

    def read(self):
        while self._fill(): pass
        return super(FastIO, self).read()

    def readline(self):
        while self.newlines == 0:
            s = self._fill();
            self.newlines = s.count(b"\n") + (not s)
        self.newlines -= 1
        return super(FastIO, self).readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.getvalue())
            self.truncate(0), self.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        if py2:
            self.write = self.buffer.write
            self.read = self.buffer.read
            self.readline = self.buffer.readline
        else:
            self.write = lambda s: self.buffer.write(s.encode('ascii'))
            self.read = lambda: self.buffer.read().decode('ascii')
            self.readline = lambda: self.buffer.readline().decode('ascii')


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip('\r\n')

""" main function"""

if __name__ == '__main__':
    # main()
    threading.Thread(target=main).start()