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".
// @jagrit_07;
import java.util.*;
import java.math.*;
import java.io.*;
import java.lang.*;

public class Main {
    
	public static void main(String[] args) {
        FastReader sc = new FastReader();
        int n = sc.nextInt();
        int a[][] = new int[n][2];
        for(int i=0;i<n;i++)
        {
            a[i][0]=sc.nextInt();
            a[i][1]=sc.nextInt();
        }
        
        //Now I want element with degree equal to 1;
        List<String> edges = new ArrayList<>();
        
        //Okay so I will add the edges nodes with degree one to queue;
        //And will dec degree accordingly and see if anyones degree become 1 so will add it to queue;
        //I will add the index in the queue;
        Queue<Integer> q=new LinkedList<>();
        for(int i=0;i<n;i++)
        {
            if(a[i][0]==1)
            {
                q.add(i); //this index I need;
            }
        }
        
        while(q.size()>0)
        {
            int u = q.poll(); //this is u
            if(a[u][0]<=0) continue;
            //Now this element has 1 edge right;
            //This is the edge;
            String edge = u + " " + a[u][1];
            edges.add(edge);
            
            //Now I need to decrease the degree of a[ind][1] and also xor this value to its a[ind][1] so that this edge gets cancelled;
            
            int v = a[u][1];
            
            //Decreased its indegree;
            a[v][0]--;
            a[v][1] = a[v][1] ^ u; //removing this edge;
            if(a[v][0]==1)
            {
                q.add(v);
            }
        }
        System.out.println(edges.size());
        for(String s: edges) System.out.println(s);
        //System.out.println(Arrays.toString(edges.toArray()));
    }


    public static int bs(int num[], long x)
    {
        int low=0;
        int high=num.length-1;
        int ans=-1;
        while(low<=high)
        {
            int mid = low + (high-low)/2;
            if(num[mid]==x)
            {
                ans = mid;
                //break;
                low = mid+1;
            }
            else if(num[mid]<x)
            {
               // ans=mid;
                ans = mid;
                low=mid+1;
            }
            else
            {
                //ans=mid;
                high=mid-1;
            }
        }
        return ans;
    }
    
    
    
    
    
    
    
    
    
    public static List<Integer> solve(char ch[], int split)
    {
        long sum = 0;
        List<Integer> l1 = new ArrayList<>();
        for(int i=0;i<split && i<ch.length;i++)
        {
            l1.add(ch[i]-'0');
        }
        List<Integer> l2 = new ArrayList<>();
        for(int i=split;i<ch.length;i++)
        {
            l2.add(ch[i]-'0');
        }
        //System.out.println(Arrays.toString(l1.toArray()));
        //System.out.println(Arrays.toString(l2.toArray()));
        int carry = 0;
        int i = l1.size()-1;
        int j = l2.size()-1;
        List<Integer> list = new ArrayList<>();
        while(i>=0 || j>=0)
        {
            int e1 = (i>=0) ? l1.get(i) : 0;
            int e2 = (j>=0) ? l2.get(j) : 0;
            i--;
            j--;
            int e3 = e1 + e2 + carry;
            if(e3>9)
            {
                list.add(e3%10);
                carry = 1;
            }
            else
            {
                list.add(e3);
                carry = 0;
            }
        }
        if(carry>0)
        {
            list.add(1);
        }
       // System.out.println(Arrays.toString(l2.toArray()));
        Collections.reverse(list);
        return list;
    }
    
    //Comparing two list;
    public static List<Integer> comp(List<Integer> l1, List<Integer> l2)
    {
        if(l1.size()<l2.size()) return l1;
        if(l2.size()<l1.size()) return l2;
        
        for(int i=0;i<l1.size();i++)
        {
            if(l1.get(i)<l2.get(i))
            {
                return l1;
            }
            else if(l2.get(i)<l1.get(i))
            {
                return l2;
            }
        }
        return l2;
    }
   
    
   
    
 
    /* to find (x^y)%p in O(log y) */
    static long power(long x, long y, int p) 
    { 
        // Initialize result 
        long ans = 1;      
         
        // Update x if it is more   
        // than or equal to p 
        x = x % p;  
  
       if (x == 0) return 0; // In case x is divisible by p;    
  
        while (y > 0) 
        { 
            if((y & 1)==1) 
            {
                 ans = ((long)(ans * x))% p;    
            }
            y = y >> 1;  //y/2; 
            x = ((long)(x * x) )% p;  
        } 
        return ans; 
    } 
    
    
    
    
    
    public static void printList(List<Integer> list)
    {
        StringBuffer bf = new StringBuffer("");
        for(int i: list) bf.append(i+"");
        System.out.println(bf.toString());
    }
    
    
    
    
    
    public static int sum(int a[])
    {
        int sum=0;
        for(int i: a) sum=sum+i;
        return sum;
    }
    
  
    public static int getMax(int a[])
    {
        int m=Integer.MIN_VALUE;
        for(int i: a) m=Math.max(i,m);
        return m;
    }
    
    static class Pair
    {
        int x;
        int y;

        //int i;
        public Pair(int x, int  y)
        {
            this.x=x;
            this.y=y;
        }
    }
    static class Comp implements Comparator<Pair>
    {
        public int compare(Pair p1, Pair p2)
        {
            if(p1.y!=p2.y)
            {
                return p2.y - p1.y;
            }
            return p2.x - p1.x;
        }
    }
    
    
    
    public static int partition(int arr[], int low, int high) 
    { 
        int pivot = arr[high];  
        int i = (low-1);
        for (int j=low; j<high; j++) 
        { 
            if (arr[j] < pivot) 
            { 
                i++; 
                int temp = arr[i]; 
                arr[i] = arr[j]; 
                arr[j] = temp; 
            } 
        } 
        int temp = arr[i+1]; 
        arr[i+1] = arr[high]; 
        arr[high] = temp; 
  
        return i+1; 
    } 
    public static void sort(int arr[], int low, int high) 
    { 
        if (low < high) 
        { 
            int pi = partition(arr, low, high); 
            sort(arr, low, pi-1); 
            sort(arr, pi+1, high); 
        } 
    } 
    
    
    
    
    
    
    static long gcd(long a,long b)
    {
        if(b==0) return a;
        return gcd(b,a%b);
    }
    
    
    static void sort(int[] a) {
		ArrayList<Integer> l=new ArrayList<>();
		for (int i:a) l.add(i);
		Collections.sort(l);
		for (int i=0; i<a.length; i++) a[i]=l.get(i);
	}
    static void sort(long[] a) {
		ArrayList<Long> l=new ArrayList<>();
		for (long i:a) l.add(i);
		Collections.sort(l);
		for (int i=0; i<a.length; i++) a[i]=l.get(i);
	}
    
	static class FastReader {
		BufferedReader br;
		StringTokenizer st;
 
		public FastReader() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}
 
		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (final IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

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

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

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (final IOException e) {
				e.printStackTrace();
			}
			return str;
		}

		int[] nextIntArray(final int n) {
			final int a[] = new int[n];
			for (int i = 0; i < n; i++) {
				a[i] = Integer.parseInt(next());
			}
			return a;
		}

	}	
}
//private ArrayList<String[]> action = new ArrayList<String[]>();


//Templates for Comparator and Classes - @jagrit_07
/*
Arrays.sort(newEmployees, new Comparator<Employee>() {
    @Override
    public int compare(Employee emp1, Employee emp2) {
        return emp1.getName().compareTo(emp2.getName());
    }
});
class Pair
	{
		
		long i; //index;
		long l; //left;
		long c; //cost;
		public Pair(long x,long y,long z)
		{
			this.i=x;
			this.l=y;
			this.c=z;
		}
		public String toString()
		{
			return this.i+" "+this.l+" "+this.c;
		}
	}
class Comp implements Comparator<Pair>
	{
		public int compare(Pair p1, Pair p2)
		{
			if(p1.c!=p2.c)
			{
				return (int)(p1.c-p2.c); //sort acc to cost;
			}
			else{
				return (int)(p1.i-p2.i); //sort acc to index;
			}
			
		}
	}
*/

/*
System.out.println( 
      String.format("%.5f", b)); 
  HashMap - Put template - d.put(a1,d.getOrDefault(a1,0)+1);
  for (Map.Entry<String, Integer> entry : map.entrySet()) {
        System.out.println(entry.getKey() + ":" + entry.getValue());
    }
  Deque<String> deque 
            = new LinkedList<String>();  
            
  List<Integer> c[] = new ArrayList[3];          
*/