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".
//pakage joney_000[let_me_start]
 
import java.util.*;
import java.lang.*;
import java.io.*;
import java.math.*;
 /******************** Main Class ***********************/

public class C
{   
    public static InputStream inputStream = System.in;
    public static OutputStream outputStream = System.out;
    public static FastReader in = new FastReader(inputStream);;
    public static PrintWriter out = new PrintWriter(outputStream);;
    /*
    Overhead [Additional Temporary Strorage]
    */
    public static int tempints[] = new int[100005];
    public static long templongs[] = new long[100005];
    public static double tempdoubles[] = new double[100005];
    public static char tempchars[] = new char[100005];
    public static long mod = 1000000000+7;
    
        public static void main(String[] args) throws java.lang.Exception{
        //let_me_start 
         int n = i(); //int m = i();
         
         //int arr[] = is((int)n);
         //String ans = "NO";
         //long ans=0;
         int d[] = new int[n+1];
         int x[]= new int[n+1];
         
        LinkedList <Integer>ll = new LinkedList();
         for(int i=0;i<n;i++){
          d[i]=i();x[i]=i();
          if(d[i]==1)ll.add(i);
         }
        // out.write(""+(n-1)+"\n");
         int cnt=0;
         int ans[][] = new int[n+1][3];
         while(!ll.isEmpty()){
           int a = ll.removeFirst();
           if(d[a]==0)continue;
           d[a]--;
           if(d[x[a]]==0)continue;
           d[x[a]]--;
           x[x[a]]^=a;
           if(d[x[a]]==1){ll.add(x[a]);
           
           }
           cnt++;
           ans[cnt][0]=a;
           ans[cnt][1]=x[a];
           
           ///out.write(""+a +" "+x[a]+"\n");
         }
        out.write(""+cnt+"\n");
        for(int i=1;i<=cnt;i++)out.write(""+ans[i][0]+" "+ans[i][1]+"\n");
        out.flush();
        return;
        }    
    
         


//****************************** Utilities ***********************//

 public static boolean isPrime(long n)throws Exception{
  if(n==1)return false;
  if(n<=3)return true;
  if(n%2==0)return false;
  for(int i=2 ;i <= Math.sqrt(n); i++){
   if(n%i==0)return false;
  }
  return true;
 }
 // sieve
 public static int[] primes(int n)throws Exception{       // for(int i=1;i<=arr.length-1;i++)out.write(""+arr[i]+" ");
  boolean arr[] = new boolean[n+1];
  Arrays.fill(arr,true);
  for(int i=1;i<=Math.sqrt(n);i++){
    if(!arr[i])continue;
    for(int j = 2*i ;j<=n;j+=i){
        arr[i]=false;
    }
  }
  LinkedList<Integer> ll = new LinkedList<Integer>();
  for(int i=1;i<=n;i++){
   if(arr[i])ll.add(i);
  }
  n = ll.size();
  
  int primes[] = new int[n+1];
  for(int i=1;i<=n;i++){
    primes[i]=ll.removeFirst();
  }
  return primes;
 }
 public static long gcd (long a , long b)throws Exception{
  if(b==0)return a;
  return gcd(b , a%b);
 }
 public static long lcm (long a , long b)throws Exception{
  if(a==0||b==0)return 0;
  return (a*b)/gcd(a,b);
 }
 public static long mulmod(long a , long b ,long mod)throws Exception{
   if(a==0||b==0)return 0;
   if(b==1)return a;
   long ans = mulmod(a,b/2,mod);
   ans = (ans*2)% mod;
   if(b%2==1)ans = (a + ans)% mod;
   return ans;
 }
 public static long pow(long a , long b ,long mod)throws Exception{
   if(b==0)return 1;
   if(b==1)return a;
   long ans = pow(a,b/2,mod);
   ans = (ans * ans)% mod;
   if(b%2==1)ans = (a * ans)% mod;
   return ans;
 }
 // 20*20   nCr Pascal Table
 public static long[][] ncrTable()throws Exception{
  long ncr[][] = new long[21][21];
  for(int i=0 ;i<=20 ;i++){ncr[i][0]=1;ncr[i][i]=1;}
  for(int j=0;j<=20 ;j++){
   for(int i=j+1;i<= 20 ;i++){
    ncr[i][j] = ncr[i-1][j]+ncr[i-1][j-1];
   }
  }
  return ncr;
 }
//*******************************I/O******************************//    
public static int i()throws Exception{
 //return Integer.parseInt(br.readLine().trim());
 return in.nextInt();
}
public static int[] is(int n)throws Exception{
  //int arr[] = new int[n+1];
  for(int i=1 ; i <= n ;i++)tempints[i] = in.nextInt();  
 return tempints;
}
public static long l()throws Exception{
 return in.nextLong();
}
public static long[] ls(int n)throws Exception{
  for(int i=1 ; i <= n ;i++)templongs[i] = in.nextLong();  
 return templongs;
}

public static double d()throws Exception{
 return in.nextDouble();
}
public static double[] ds(int n)throws Exception{
  for(int i=1 ; i <= n ;i++)tempdoubles[i] = in.nextDouble();  
 return tempdoubles;
}
public static char c()throws Exception{
 return in.nextCharacter();
}
public static char[] cs(int n)throws Exception{
  for(int i=1 ; i <= n ;i++)tempchars[i] = in.nextCharacter();  
 return tempchars;
}
public static String s()throws Exception{
 return in.nextLine();
}
public static BigInteger bi()throws Exception{
 return in.nextBigInteger();
}
//***********************I/O ENDS ***********************//
//*********************** 0.3%f [precision]***********************//
/* roundoff upto 2 digits 
   double roundOff = Math.round(a * 100.0) / 100.0;
                    or
   System.out.printf("%.2f", val);
                    
*/
/*
  print upto 2 digits after decimal
  val = ((long)(val * 100.0))/100.0;
  
*/
}

class FastReader{

    private boolean finished = false;

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

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

    public long nextLong(){
        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 nextString(){
        int c = read ();
        while (isSpaceChar (c))
            c = read ();
        StringBuilder res = new StringBuilder ();
        do{
            res.appendCodePoint (c);
            c = read ();
        } while (!isSpaceChar (c));
        return res.toString ();
    }

    public boolean isSpaceChar(int c){
        if (filter != null){
            return filter.isSpaceChar (c);
        }
        return isWhitespace (c);
    }

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

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

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

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

    public BigInteger nextBigInteger(){
        try{
            return new BigInteger (nextString ());
        } catch (NumberFormatException e){
            throw new InputMismatchException ();
        }
    }

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

    public double nextDouble(){
        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, nextInt ());
            }
            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, nextInt ());
                }
                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 nextString ();
    }

    public SpaceCharFilter getFilter(){
        return filter;
    }

    public void setFilter(SpaceCharFilter filter){
        this.filter = filter;
    }

    public interface SpaceCharFilter{
        public boolean isSpaceChar(int ch);
    }
}
 /******************** Pair class ***********************/
 
 class Pair implements Comparable<Pair>{
 public int a;
 public int b;
 public Pair(){
  this.a = 0;
  this.b = 0;
 }
 public Pair(int a,int b){
  this.a = a;
  this.b = b;
 }
 public int compareTo(Pair p){
  if(this.a==p.a){
   return this.b-p.b;  
  }
  return this.a-p.a; 
 }
 public String toString(){
  return "a="+this.a+" b="+this.b;
 }
 
} 
