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".
/* Codeforces Template */

import static java.util.Arrays.sort;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Queue;
import java.util.Random;
import java.util.StringTokenizer;

public class Main {
	public void run() throws IOException {
		in = new BufferedReader(new InputStreamReader(System.in));
		out = new PrintWriter(System.out);
		while (!EOF())
			solve();
		out.close();
		in.close();
	}

	void solve() throws IOException {
		int n = nextInt();
		int[] degrees = new int[n];
		int[] xors = new int[n];
		Queue<Integer> queue = new LinkedList<Integer>();
		for (int i = 0; i < n; i++) {
			degrees[i] = nextInt();
			xors[i] = nextInt();
			if (degrees[i] == 1) {
				queue.add(i);
			}
		}
		List<int[]> answers = new ArrayList<int[]>();
		while (!queue.isEmpty()) {
			int v = queue.poll();
			int d = xors[v];
			if (degrees[v] == 1) {
				answers.add(new int[] { v, d });
				xors[d] = xors[d] ^ v;
				degrees[d]--;
				if (degrees[d] == 1) {
					queue.add(d);
				}
			}
		}

		System.out.println(answers.size());
		for (int i = 0; i < answers.size(); i++) {
			int[] a = (int[]) answers.get(i);
			System.out.println(a[0] + " " + a[1]);
		}
	}

	static long initTime;
	static final Random rnd = new Random(7777L);
	static boolean writeLog = false;

	public static void main(String[] args) throws IOException {
		initTime = System.currentTimeMillis();
		try {
			writeLog = "true".equals(System.getProperty("LOCAL_RUN_7777"));
		} catch (SecurityException e) {
		}
		new Thread(null, new Runnable() {
			@Override
			public void run() {
				try {
					try {
						if (new File("input.txt").exists())
							System.setIn(new FileInputStream("input.txt"));
					} catch (SecurityException e) {
					}
					long prevTime = System.currentTimeMillis();
					new Main().run();
					log("Total time: "
							+ (System.currentTimeMillis() - prevTime) + " ms");
					log("Memory status: " + memoryStatus());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}, "1", 1L << 24).start();
	}

	/***************************************************************
	 * Input
	 **************************************************************/
	BufferedReader in;
	PrintWriter out;
	StringTokenizer st = new StringTokenizer("");

	String nextToken() throws IOException {
		while (!st.hasMoreTokens())
			st = new StringTokenizer(in.readLine());
		return st.nextToken();
	}

	int nextInt() throws IOException {
		return Integer.parseInt(nextToken());
	}

	long nextLong() throws IOException {
		return Long.parseLong(nextToken());
	}

	double nextDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}

	int[] nextIntArray(int size) throws IOException {
		int[] ret = new int[size];
		for (int i = 0; i < size; i++)
			ret[i] = nextInt();
		return ret;
	}

	long[] nextLongArray(int size) throws IOException {
		long[] ret = new long[size];
		for (int i = 0; i < size; i++)
			ret[i] = nextLong();
		return ret;
	}

	double[] nextDoubleArray(int size) throws IOException {
		double[] ret = new double[size];
		for (int i = 0; i < size; i++)
			ret[i] = nextDouble();
		return ret;
	}

	String nextLine() throws IOException {
		st = new StringTokenizer("");
		return in.readLine();
	}

	boolean EOF() throws IOException {
		while (!st.hasMoreTokens()) {
			String s = in.readLine();
			if (s == null)
				return true;
			st = new StringTokenizer(s);
		}
		return false;
	}

	/***************************************************************
	 * Output
	 **************************************************************/
	void printRepeat(String s, int count) {
		for (int i = 0; i < count; i++)
			out.print(s);
	}

	void printArray(int[] array) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0)
				out.print(' ');
			out.print(array[i]);
		}
		out.println();
	}

	void printArray(long[] array) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0)
				out.print(' ');
			out.print(array[i]);
		}
		out.println();
	}

	void printArray(double[] array) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0)
				out.print(' ');
			out.print(array[i]);
		}
		out.println();
	}

	void printArray(double[] array, String spec) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0)
				out.print(' ');
			out.printf(Locale.US, spec, array[i]);
		}
		out.println();
	}

	void printArray(Object[] array) {
		if (array == null || array.length == 0)
			return;
		boolean blank = false;
		for (Object x : array) {
			if (blank)
				out.print(' ');
			else
				blank = true;
			out.print(x);
		}
		out.println();
	}

	@SuppressWarnings("rawtypes")
	void printCollection(Collection collection) {
		if (collection == null || collection.isEmpty())
			return;
		boolean blank = false;
		for (Object x : collection) {
			if (blank)
				out.print(' ');
			else
				blank = true;
			out.print(x);
		}
		out.println();
	}

	/***************************************************************
	 * Utility
	 **************************************************************/
	static String memoryStatus() {
		return (Runtime.getRuntime().totalMemory()
				- Runtime.getRuntime().freeMemory() >> 20)
				+ "/" + (Runtime.getRuntime().totalMemory() >> 20) + " MB";
	}

	static void checkMemory() {
		System.err.println(memoryStatus());
	}

	static long prevTimeStamp = Long.MIN_VALUE;

	static void updateTimer() {
		prevTimeStamp = System.currentTimeMillis();
	}

	static long elapsedTime() {
		return (System.currentTimeMillis() - prevTimeStamp);
	}

	static void checkTimer() {
		System.err.println(elapsedTime() + " ms");
	}

	static void chk(boolean f) {
		if (!f)
			throw new RuntimeException("Assert failed");
	}

	static void chk(boolean f, String format, Object... args) {
		if (!f)
			throw new RuntimeException(String.format(format, args));
	}

	static void log(String format, Object... args) {
		if (writeLog)
			System.err.println(String.format(Locale.US, format, args));
	}

	static void swap(int[] a, int i, int j) {
		int tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}

	static void swap(long[] a, int i, int j) {
		long tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}

	static void swap(double[] a, int i, int j) {
		double tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}

	static void shuffle(int[] a, int from, int to) {
		for (int i = from; i < to; i++)
			swap(a, i, rnd.nextInt(a.length));
	}

	static void shuffle(long[] a, int from, int to) {
		for (int i = from; i < to; i++)
			swap(a, i, rnd.nextInt(a.length));
	}

	static void shuffle(double[] a, int from, int to) {
		for (int i = from; i < to; i++)
			swap(a, i, rnd.nextInt(a.length));
	}

	static void shuffle(int[] a) {
		if (a == null)
			return;
		shuffle(a, 0, a.length);
	}

	static void shuffle(long[] a) {
		if (a == null)
			return;
		shuffle(a, 0, a.length);
	}

	static void shuffle(double[] a) {
		if (a == null)
			return;
		shuffle(a, 0, a.length);
	}

	static long[] getPartialSums(int[] a) {
		final long[] sums = new long[a.length + 1];
		for (int i = 0; i < a.length; i++)
			sums[i + 1] = sums[i] + a[i];
		return sums;
	}

	static long[] getPartialSums(long[] a) {
		final long[] sums = new long[a.length + 1];
		for (int i = 0; i < a.length; i++)
			sums[i + 1] = sums[i] + a[i];
		return sums;
	}

	static int[] getOrderedSet(int[] a) {
		final int[] set = Arrays.copyOf(a, a.length);
		if (a.length == 0)
			return set;
		shuffle(set);
		sort(set);
		int k = 1;
		int prev = set[0];
		for (int i = 1; i < a.length; i++) {
			if (prev != set[i]) {
				set[k++] = prev = set[i];
			}
		}
		return Arrays.copyOf(set, k);
	}

	static long[] getOrderedSet(long[] a) {
		final long[] set = Arrays.copyOf(a, a.length);
		if (a.length == 0)
			return set;
		shuffle(set);
		sort(set);
		int k = 1;
		long prev = set[0];
		for (int i = 1; i < a.length; i++) {
			if (prev != set[i]) {
				set[k++] = prev = set[i];
			}
		}
		return Arrays.copyOf(set, k);
	}
}
