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".
//################################################################################################################
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import java.io.Writer;
import java.math.BigInteger;
import java.util.HashSet;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.Set;

//################################################################################################################
public class R285D2C {
	public static void main(String[] args) {
		R285D2C c = new R285D2C();
		InputReader1 reader = c.new InputReader1(System.in);
		OutputWriter writer = c.new OutputWriter(System.out);
		// think about the brute-force approach
		// read input
		// code boundary conditions
		int n = reader.readInt();
		int[] deg = new int[n];
		int[] sval = new int[n];
		long sum = 0;
		LinkedList<Integer> d1nodes = new LinkedList<Integer>();
		for (int i = 0; i < n; i++) {
			int d = reader.readInt();
			int s = reader.readInt();
			deg[i] = d;
			sval[i] = s;
			sum += deg[i];
			if (deg[i] == 1) {
				d1nodes.add(i);
			}
		}
		Set<String> edges = new HashSet<String>();
		while (!d1nodes.isEmpty()) {
			int id = d1nodes.removeFirst();
			int d = deg[id];
			int s = sval[id];
			if (d == 0) {
				continue;
			}
			// id & s make an edge
			edges.add(id + " " + s);
			// System.out.println(id + " " + s);
			// update
			sval[s] = sval[s] ^ id;
			deg[s]--;
			if (deg[s] == 1) {
				d1nodes.add(s);
			}
			// System.out.println("d1nodes:");
			// for (int i = 0; i < d1nodes.size(); i++) {
			// System.out.print(d1nodes.get(i) + " ");
			// }
			// System.out.println();
			// System.out.println("deg:");
			// for (int i = 0; i < deg.length; i++) {
			// System.out.print(deg[i] + " ");
			// }
			// System.out.println();
			// System.out.println("sval:");
			// for (int i = 0; i < sval.length; i++) {
			// System.out.print(sval[i] + " ");
			// }
			// System.out.println();
		}
		writer.println(edges.size());
		for (String edge : edges) {
			writer.println(edge);
		}
		reader.close();
		writer.close();
	}

	class InputReader1 {
		// private final boolean finished = false;

		private InputStream stream;
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;

		public InputReader1(InputStream stream) {
			this.stream = stream;
		}

		public int read() {
			if (numChars == -1)
				throw new InputMismatchException();
			if (curChar >= numChars) {
				curChar = 0;
				try {
					numChars = stream.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}

		public int peek() {
			if (numChars == -1)
				return -1;
			if (curChar >= numChars) {
				curChar = 0;
				try {
					numChars = stream.read(buf);
				} catch (IOException e) {
					return -1;
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar];
		}

		public int readInt() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			int res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public long readLong() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public String readString() {
			int length = readInt();
			if (length < 0)
				return null;
			byte[] bytes = new byte[length];
			for (int i = 0; i < length; i++)
				bytes[i] = (byte) read();
			try {
				return new String(bytes, "UTF-8");
			} catch (UnsupportedEncodingException e) {
				return new String(bytes);
			}
		}

		public boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private String readLine0() {
			StringBuffer buf = new StringBuffer();
			int c = read();
			while (c != '\n' && c != -1) {
				if (c != '\r')
					buf.appendCodePoint(c);
				c = read();
			}
			return buf.toString();
		}

		public String readLine() {
			String s = readLine0();
			while (s.trim().length() == 0)
				s = readLine0();
			return s;
		}

		public String readLine(boolean ignoreEmptyLines) {
			if (ignoreEmptyLines)
				return readLine();
			else
				return readLine0();
		}

		public BigInteger readBigInteger() {
			try {
				return new BigInteger(readString());
			} catch (NumberFormatException e) {
				throw new InputMismatchException();
			}
		}

		public char readCharacter() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			return (char) c;
		}

		public double readDouble() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			double res = 0;
			while (!isSpaceChar(c) && c != '.') {
				if (c == 'e' || c == 'E')
					return res * Math.pow(10, readInt());
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			}
			if (c == '.') {
				c = read();
				double m = 1;
				while (!isSpaceChar(c)) {
					if (c == 'e' || c == 'E')
						return res * Math.pow(10, readInt());
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					m /= 10;
					res += (c - '0') * m;
					c = read();
				}
			}
			return res * sgn;
		}

		public boolean isExhausted() {
			int value;
			while (isSpaceChar(value = peek()) && value != -1)
				read();
			return value == -1;
		}

		public String next() {
			return readString();
		}

		public boolean readBoolean() {
			return readInt() == 1;
		}

		public void close() {
			try {
				stream.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	class OutputWriter {
		private final PrintWriter writer;

		public OutputWriter(OutputStream stream) {
			writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
					stream)));
		}

		public OutputWriter(Writer writer) {
			this.writer = new PrintWriter(writer);
		}

		public void println(String s) {
			writer.println(s);
		}

		public void println(int x) {
			writer.println(x);
		}

		public void print(int x) {
			writer.print(x);
		}

		public void println(long x) {
			writer.println(x);
		}

		public void printSpace() {
			writer.print(" ");
		}

		public void close() {
			writer.close();
		}
	}
}
