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.awt.List;
import java.io.*;
import java.util.*;




/*
 * kishan Thesiya
 * daiict
 * kishanthesiya25@gmail.com
 * 
 * 
 */


public class Main {
	FastScanner in;
	PrintWriter out;

	static ArrayList[] adj;
	static ArrayList[] obj;
	static int n;
	static int x;
	static int c;
	static int visited[];
	static int Ad[];
	static int Bd[];
 
	static int brr[][];
	static int a;
	static int b;
	static int parent[];
	static int size[];
	long mod = (long)1e9+7;
	static int ds[];
	
//Segment tree do not delete
	static int arr[];	
	static int d[];
	static int h;
// till now	

	class Segment{
	  private  int Lazy[];
	 
	    void ans(){
	   
	    	
	     int x = (int) (Math.ceil(Math.log(100000) / Math.log(2)));
	        int max_size = 2 * (int) Math.pow(2, x) - 1;
	        int SegTree[][] = new int[max_size][3]; // allocate memory
	        Lazy = new int[max_size];
	        
	       String s = in.next();
	       n = s.length();
	        
	        int q=in.nextInt();
	        

	        build_ST(0, 0, n - 1,SegTree,s);
	  
	        for(int i = 0;i<2*n;i++){
	        	System.out.println(SegTree[i][0] + " " + SegTree[i][1] + " " + SegTree[i][2] + " " + (i+n));
	        }
	        
	        while(q-->0)
	        {
	          int a = in.nextInt();
	          int b = in.nextInt();
	          
	          int d = getMaxST(0, n-1, a-1, b-1, 0,SegTree);
	          System.out.println(d);
	        }
	    }
	    
	    
	 void build_ST(int node, int start, int end,int SegTree[][],String s) 
	   {
	    if(start > end) return; 
	  	if(start == end) { 
	  		System.out.println(start + " " + node);
	  		if(s.charAt(start) == '('){
	  			SegTree[node][1] = 1;
	  		}else{
	  			SegTree[node][2] = 1;
	  		}
	  		
			return;
		}
		int mid=(start + end) >> 1;
		build_ST(node*2+1, start, mid, SegTree,s); // Init left child
		build_ST(node*2+2, 1+mid, end,SegTree,s); // Init right child
		
		// Init root value
		
		int t = Math.min(SegTree[2*node + 1][1], SegTree[2*node+2][2]);
		
		SegTree[node][0] = SegTree[2*node+1][0] + SegTree[2*node+2][0] + 2*t;
		SegTree[node][1] = SegTree[2*node+1][1] + SegTree[2*node+2][1] - t;
		SegTree[node][2] = SegTree[2*node+1][2] + SegTree[2*node+2][2] - t;
		
		
	}
//	    void update_ST(int node, int start, int end, int l, int r, int diff) {
//	        if(Lazy[node] != 0)
//	        {
//	            int pending = Lazy[node];
//	            
//	            SegTree[node] -= pending;
//	            
//	           if(start != end)
//	            {
//	                Lazy[2 * node + 1] += Lazy[node];
//	                Lazy[2 * node + 2] += Lazy[node];
//	            }
//	           Lazy[node] = 0;
//	        }
//	        if (end < l || start > r)
//	            return ;
//	          if(l<=start && end<=r)
//	        {
//	        	  SegTree[node] -= diff;
//	               
//	            if(start != end) // not a leaf
//	            {
//	                Lazy[2*node+1] += diff;
//	                Lazy[2*node+2] += diff;
//	            }
//	            return;
//	        }
//	        int mid=(start + end) >> 1;
//		update_ST(node*2+1, start, mid, l, r, diff); // Updating left child
//		update_ST(2+node*2, 1+mid, end, l, r, diff); // Updating right child
//	
//		SegTree[node] = Math.max(SegTree[node*2+1],SegTree[node*2+2]); 
//		
//	    }
	      int getMaxST(int start, int end, int l, int r, int node,int SegTree[][])
	    {
//	    	  if(Lazy[node] != 0)
//		        {
//		            int pending = Lazy[node];
//		            
//		            SegTree[node] -= pending;
//		            
//		           if(start != end)
//		            {
//		                Lazy[2 * node + 1] += Lazy[node];
//		                Lazy[2 * node + 2] += Lazy[node];
//		            }
//		           Lazy[node] = 0;
//		        }
	        if (l <= start && r >= end)
	            return SegTree[node][0];
	         if (end < l || start > r)
	            return 0;
	        int mid = (start + end) >> 1;
	        return getMaxST(start, mid, l, r, 2 * node + 1,SegTree)+getMaxST(mid + 1, end, l, r, 2 * node + 2,SegTree);
	    }
	      
	}
	
	static class Pair implements Comparable<Pair>
    {
		int x;int y;int i;
		
        Pair(int l, int m,int i) {
          
        	this.x = l;
            this.y = m;
            this.i = i;
           
        }
        
        public int compareTo(Pair p) {
            if (this.x == p.x) {
                return (int)(this.y-p.y);
            }
            return (int)(this.x - p.x);
        }
        
        public String toString(){
        	return x + " " + y;
        }
        
        public boolean equals(Object o){
        	if(o instanceof Pair){
        		Pair a = (Pair)o;
        		
        		return this.x == a.x && this.y == a.y;
        	}
        	
        	return false;
        }
        
        public int hashCode(){
        	return new Long(x).hashCode()*31 + new Long(y).hashCode();
        }
    
    }
    
	
	public static int gcd(int a, int b) {
		if (b == 0) return a;
		return gcd(b, a%b);
	}
	
	public static int find(int i){
		if(parent[i] != i){
			return find(parent[i]);
		}
		
		return i;
	}
	
	
	public static void union(int a,int b){
		int ra = find(a);
		int rb = find(b);

		if(ra!= rb){
			if(size[ra] > size[rb]){
				parent[rb] =ra;
				size[ra] += size[rb];
			}else{
				parent[ra] = rb;
				size[rb] += size[ra];
			}
		}
		
	}

	public static void apply(int p,int value){
		arr[p] += value;
		
		if(p < n){
			d[p] += value;
		}
	}
	
	public static void push(int p){
		
		for(int s = h;s>0;s--){
			int i = p >> s;
			
			if(d[i] != 0){
				apply(i<<1,d[i]);
				apply(i<<1|1,d[i]);
				d[i] = 0;
			}		
		}		
	}
	
	public static void lazybuild(int p){
		while(p > 1){
			p>>=1;
			arr[p] = Math.max(arr[p<<1], arr[p<<1|1]) + d[p];
		}
	}
	
	public static void lazyupdate(int l,int r,int value){
		l += n; r += n+1;		
		int l0 = l;int r0 = r;
		for(;l<r; l>>=1,r>>=1){
			if((l&1) == 1){
				apply(l++,value);
			}
			
			if((r&1) == 1){
				apply(--r,value);
			}
		}
		lazybuild(l0);
		lazybuild(r0-1);
			
	}
	
	public static int lazyquery(int l,int r){
		l += n;r += n+1;
		push(l);push(r-1);
		int res = (int)-2e9;
		
		for(;l<r;l>>=1,r>>=1){
			if((l&1) == 1){
				res = Math.max(arr[l++], res);
			}
			
			if((r&1) == 1){
				res = Math.max(arr[--r], res);
			}
		}
		
		return res;
	}
	
	public static void build(){
		
		for(int i = n-1;i>0;i--){
			arr[i] = Math.max(arr[i<<1], arr[i<<1|1]);
		}
		
	}
	
	public static void update(int p,int v){
		
		for(arr[p += n] = v; p>0; p>>=1 ){
			arr[p>>1] = Math.max(arr[p], arr[p^1]);
		}
		
	}
	
	public static int query(int l,int r){
		
		r++;
		int max = 0;
		for(l +=n,r +=n;l<r; l>>=1,r>>=1 ){
			
		
			
			if((l&1) == 1){
				max = Math.max(max, arr[l++]);
			}
			
			if((r&1) == 1){
				max = Math.max(max, arr[--r]);
			}
		}
		
		return max;
	}
	
	void multiply(long a[][], long b[][])
	{
	    
	    long mul[][] = new long[3][3];
	    for (int i = 0; i < 3; i++)
	    {
	        for (int j = 0; j < 3; j++)
	        {
	            mul[i][j] = 0;
	            for (int k = 0; k < 3; k++)
	                mul[i][j] = (int) ((mul[i][j] + (a[i][k]*b[k][j])%mod)%mod);
	        }
	    }
	 
	  
	    for (int i=0; i<3; i++)
	        for (int j=0; j<3; j++)
	            a[i][j] = mul[i][j];  
	}
	
	long power(long F[][], long n)
	{
	    long M[][] = {{1,1,0}, {1,0,0}, {0,1,0}};
	 
	    if (n==1)
	        return (long) ((F[0][0] + F[0][1])%mod);
	 
	    power(F, n/2);
	 
	    multiply(F, F);
	 
	    if (n%2 != 0)
	        multiply(F, M);
	 
	    return (long) ((F[0][0] + F[0][1])%mod) ;
	}
	 
	
	long findNthTerm(long n)
	{
	    long[][] F = {{1,1,0}, {1,0,0}, {0,1,0}} ;
	 
	    return power(F, n);
	}
	 
	
	
    void solve() throws NumberFormatException, IOException {
 
    	Scanner in = new Scanner (System.in);
		
    	int n = in.nextInt();
    	in.nextLine();
    
    	int a[] = new int[n];
    	int b[] = new int[n];
    	
    	for(int i = 0;i<n;i++){
    		a[i] = in.nextInt();
    		b[i] = in.nextInt();
    	}
    	
    	LinkedList<Integer> q = new LinkedList();
    	
    	ArrayList<Integer> ll = new ArrayList();
    	ArrayList<Integer> rr = new ArrayList();
    	
    	for(int i = 0;i<n;i++){
    		if(a[i] == 1){
    			q.add(i);
    		}
    	}
    	
    	while(!q.isEmpty()){
    		int i = q.poll();
    		if(a[i] == 0){
    			continue;
    		}
    		
    		ll.add(i);
    		rr.add(b[i]);
    		
    		a[b[i]]--;
    		b[b[i]] = b[b[i]]^i;
    		
    		if(a[b[i]] == 1){
    			q.add(b[i]);
    		}
    		
    	}
    	
    	System.out.println(ll.size());
    	for(int i = 0;i<ll.size();i++){
    		System.out.println(ll.get(i) + " " + rr.get(i));
    	}
    	 
//    	Segment s = new Segment();
//    	
//    	s.ans();
    	
//    	n = in.nextInt();
//    	arr = new int[2*n];
//    	d = new int[n];
//    	h = (int)Math.ceil((Math.log(n)/Math.log(2)));
//    	for(int i = 0;i<n;i++){
//    		arr[i+n] = in.nextInt();
//    	}
//    	
//    	build();
    	
//    	 BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
//        
//    	 StringBuilder sb = new StringBuilder();
// 
//    	 int tests = Integer.parseInt(br.readLine().trim());
//    	  
//          while (tests-- > 0) {
//        	 
//        	
//          }
//          
//          
//          System.out.println(sb.toString());
//          
//           
//         
}

    
    
    public static String[] reverse(String arr[]){
    	
    	
    	for(int i = 0;i<arr.length/2;i++){
    		String temp = arr[i];
    		arr[i] = arr[arr.length-i-1];
    		arr[arr.length-1-i] = temp;
    	}
    	
    	return arr;
    	
    }
	
	
    
  
    private static long ans(long n){
        if(n==1) return 2;
        n+=1;
        int cnt=-1;
        while(n%2==0){
            n/=2;
            cnt++;
        }
        if(n==1){
            return (int)Math.pow(2,cnt)-1;
        }
        return -1;
    }
    public static long sum(long x){
    	return x<10?x:x%10 + sum(x/10);
    }
	
	public static long gcd(long x, long y) {
		if (x == 0)
			return y;
		else
			return gcd( y % x,x);
	}
	public static long pow(long n,long p,long m){
        long  result = 1;
         if(p==0)
           return 1;
       if (p==1)
           return n;
       while(p!=0)
       {
           if(p%2==1)
               result *= n;
           if(result>=m)
           result%=m;
           p >>=1;
           n*=n;
           if(n>=m)
           n%=m;
       }
       return result;
       }
	
	
	
	public static boolean isPrime(int n) {
		// Corner cases
		if (n <= 1)
			return false;
		if (n <= 3)
			return true;
 
		// This is checked so that we can skip 
		// middle five numbers in below loop
		if (n % 2 == 0 || n % 3 == 0)
			return false;
 
		for (int i = 5; i * i <= n; i = i + 6)
			if (n % i == 0 || n % (i + 2) == 0)
				return false;
 
		return true;
	}
	

	void run() throws NumberFormatException, IOException {
		in = new FastScanner();
		out = new PrintWriter(System.out);

		solve();

		out.close();
	}

	class FastScanner {
		BufferedReader br;
		StringTokenizer st;

		public FastScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreTokens()) {
				try {
					st = new StringTokenizer(in.br.readLine());
				} catch (IOException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}
	}
	
	class Tri{
		
		Tri[] arr;
		
		public Tri(){
			arr = new Tri[27];
		}
		
	}
	
	
 

	public static void main(String[] args) throws NumberFormatException, IOException {
		new Main().run();
	}
}


class Node{
	int a;
	int b;
	int c;
}

class MyComp implements Comparator<Node>{
    
	@Override
	public int compare(Node a,Node b) {
		
		// < == -1
		
		if(a.a < b.a){
			return -1;
		}else if(a.a > b.a){
			return 1;
		}else{
			return 0;
		}
		
		
	}
}

