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.*;
import java.util.*;
import java.math.*;
 
public class Main {
 
    FastScanner in;
    PrintWriter out;
 
    static class FastScanner {
 
        BufferedReader br;
        StringTokenizer st;
 
        FastScanner(InputStream in) {
            br = new BufferedReader(new InputStreamReader(in));
            st = null;
        }
 
        String next() throws IOException {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(br.readLine());
            return st.nextToken();
        }
 
        String nextLine() throws IOException {
            if (st == null || !st.hasMoreTokens())
                return br.readLine();
            StringBuilder result = new StringBuilder(st.nextToken());
            while (st.hasMoreTokens()) {
                result.append(" ");
                result.append(st.nextToken());
            }
            return result.toString();
        }
 
        int nextInt() throws IOException {
            return Integer.parseInt(next());
        }
 
        long nextLong() throws IOException {
            return Long.parseLong(next());
        }
 
        double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }
 
    }
 
    void run() throws IOException {
        in = new FastScanner(System.in);
        out = new PrintWriter(System.out, false);
        solve();
        out.close();
    }
 
    public static void main(String[] args) throws IOException{
        new Main().run();
    }
 
    public void printArr(int[] arr){
        for(int i = 0; i < arr.length; i++){
            out.print(arr[i] + " ");
        }
        out.println();
    }
 
    public long gcd(long a, long b){
        if(a == 0) return b;
        return gcd(b % a, a);
    }

    public boolean isPrime(long num){

        if(num == 0 || num == 1){
            return false;
        }

        for(int i = 2; i * i <= num; i++){
            if(num % i == 0){
                return false;
            }
        }

        return true;
    }

    class Pair{
        int x; int y;
        Pair(int ix, int iy){
            x = ix;
            y = iy;
        }
    }

    class Tuple{
        int x; int y; int z;
        Tuple(int ix, int iy, int iz){
            x = ix;
            y = iy;
            z = iz;
        }
    }

    int n; 
    HashMap<Integer, Tuple> map = new HashMap<Integer, Tuple>();
    Queue<Tuple> leaf = new LinkedList<Tuple>();

    public void solve() throws IOException{

        n = in.nextInt();
        for(int i = 0; i < n; i++){

            int node = i;
            int degree = in.nextInt();
            int xor = in.nextInt();
            
            //out.println(node + " " + degree + " " + xor);

            if(degree == 1){
                leaf.add(new Tuple(node, degree, xor));
            }else{
                map.put(node, new Tuple(node, degree, xor));
            }
        }

        List<Pair> res = new ArrayList<Pair>();
        int[] deletedLeaf = new int[n]; 
        while(leaf.size() != 0){

            Tuple t = leaf.poll();
            int leafNode = t.x;
            int innerNode = t.z;
            if(deletedLeaf[leafNode] == 0 && deletedLeaf[innerNode] == 0){
                res.add(new Pair(leafNode, innerNode));
                deletedLeaf[leafNode] = 1;
            }
            //out.println("Node: " + leafNode + " InnerNode "  + innerNode);

            Tuple nt = map.get(innerNode);
            
            if(nt == null){
                continue;
            }

            if(nt.y - 1 == 1){
                leaf.add(new Tuple(innerNode, 1, nt.z ^ leafNode));
            }else{
                map.put(innerNode, new Tuple(innerNode, nt.y - 1, nt.z ^ leafNode));
            }

        }

        out.println(res.size());
        for(int i = 0; i < res.size(); i++){
            out.println(res.get(i).x + " " + res.get(i).y);
        }
        
        return;
    }



}