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.lang.*;
import java.util.HashMap;
public class templ implements Runnable {
    class pair
    {
        int node,type;
        pair(int f,int s)
        {
            node=f;
            type=s;
        }
    }
    public static ArrayList<pair> g[]=new ArrayList[1000000];
    public static int vis[]=new int[1000000];
    void dfs(int n,int p)
    {
        vis[n]=1;
        for(int i=0;i<g[n].size();i++)
        {
            int u=g[n].get(i).node;
            if(vis[u]==0)
            {
                dfs(u, n);
            }
        }
    }
    int binarySearch(int arr[], int l, int r, int x)
    {
        if (r >= l)
        {
            int mid = l + (r - l)/2;
            if (arr[mid] == x)
                return mid;
            if (arr[mid] > x)
                return binarySearch(arr, l, mid-1, x);
            return binarySearch(arr, mid+1, r, x);
        }
        return -1;
    }
    void merge1(int arr[], int l, int m, int r)
    {
        int n1 = m - l + 1;
        int n2 = r - m;
        int L[] = new int [n1];
        int R[] = new int [n2];
        for (int i=0; i<n1; ++i)
            L[i] = arr[l + i];
        for (int j=0; j<n2; ++j)
            R[j] = arr[m + 1+ j];
        int i = 0, j = 0;
        int k = l;
        while (i < n1 && j < n2)
        {
            if (L[i]<=R[j])
            {
                arr[k] = L[i];
                i++;
            }
            else
            {
                arr[k] = R[j];
                j++;
            }
            k++;
        }
        while (i < n1)
        {
            arr[k] = L[i];
            i++;
            k++;
        }
        while (j < n2)
        {
            arr[k] = R[j];
            j++;
            k++;
        }
    }
    void sort1(int arr[], int l, int r)
    {
        if (l < r)
        {
            int m = (l+r)/2;
            sort1(arr, l, m);
            sort1(arr , m+1, r);
            merge1(arr, l, m, r);
        }
    }
    void merge3(int arr[],int arr1[],int arr2[], int l, int m, int r)
    {
        int n1 = m - l + 1;
        int n2 = r - m;
        int L[] = new int [n1];
        int R[] = new int [n2];
        int L1[]=new int[n1];
        int R1[]=new int[n2];
        int L2[]=new int[n1];
        int R2[]=new int[n2];
        //long L3[]=new long[n1];
        //long R3[]=new long[n2];
        for (int i=0; i<n1; ++i)
        {
            L[i] = arr[l + i];
            L1[i]=arr1[l+i];
            L2[i]=arr2[l+i];
            //L3[i]=arr3[l+i];
        }
        for (int j=0; j<n2; ++j)
        {
            R[j] = arr[m + 1+ j];
            R1[j]=arr1[m+1+j];
            R2[j]=arr2[m+1+j];
            //R3[j]=arr3[m+1+j];
        }
        int i = 0, j = 0;
        int k = l;
        while (i < n1 && j < n2)
        {
            if (L[i] <= R[j])
            {
                arr[k] = L[i];
                arr1[k]=L1[i];
                arr2[k]=L2[i];
                //arr3[k]=L3[i];
                i++;
            }
            else
            {
                arr[k] = R[j];
                arr1[k]=R1[j];
                arr2[k]=R2[j];
                //arr3[k]=R3[j];
                j++;
            }
            k++;
        }
        while (i < n1)
        {
            arr[k] = L[i];
            arr1[k]=L1[i];
            arr2[k]=L2[i];
            //arr3[k]=L3[i];
            i++;
            k++;
        }
        while (j < n2)
        {
            arr[k] = R[j];
            arr1[k]=R1[j];
            arr2[k]=R2[j];
            //arr3[k]=R3[j];
            j++;
            k++;
        }
    }
    void sort3(int arr[],int arr1[],int arr2[], int l, int r)
    {
        if (l < r)
        {
            int m = (l+r)/2;
            sort3(arr,arr1,arr2, l, m);
            sort3(arr ,arr1,arr2, m+1, r);
            merge3(arr,arr1,arr2,l, m, r);
        }
    }
    void merge2(int arr[],int arr1[],int l, int m, int r)
    {
        int n1 = m - l + 1;
        int n2 = r - m;
        int L[] = new int [n1];
        int R[] = new int [n2];
        int L1[]=new int[n1];
        int R1[]=new int[n2];
        for (int i=0; i<n1; ++i)
        {
            L[i] = arr[l + i];
            L1[i]=arr1[l+i];
        }
        for (int j=0; j<n2; ++j)
        {
            R[j] = arr[m + 1+ j];
            R1[j]=arr1[m+1+j];
        }
        int i = 0, j = 0;
        int k = l;
        while (i < n1 && j < n2)
        {
            if (L[i] <= R[j])
            {
                arr[k] = L[i];
                arr1[k]=L1[i];
                i++;
            }
            else
            {
                arr[k] = R[j];
                arr1[k]=R1[j];
                j++;
            }
            k++;
        }
        while (i < n1)
        {
            arr[k] = L[i];
            arr1[k]=L1[i];
            i++;
            k++;
        }
        while (j < n2)
        {
            arr[k] = R[j];
            arr1[k]=R1[j];
            j++;
            k++;
        }
    }
    void sort2(int arr[],int arr1[],int l, int r)
    {
        if (l < r)
        {
            int m = (l+r)/2;
            sort2(arr,arr1, l, m);
            sort2(arr ,arr1, m+1, r);
            merge2(arr,arr1,l, m, r);
        }
    }
    void merge4(int arr[],int arr1[],int arr2[],int arr3[], int l, int m, int r)
    {
        int n1 = m - l + 1;
        int n2 = r - m;
        int L[] = new int [n1];
        int R[] = new int [n2];
        int L1[]=new int[n1];
        int R1[]=new int[n2];
        int L2[]=new int[n1];
        int R2[]=new int[n2];
        int L3[]=new int[n1];
        int R3[]=new int[n2];
        for (int i=0; i<n1; ++i)
        {
            L[i] = arr[l + i];
            L1[i]=arr1[l+i];
            L2[i]=arr2[l+i];
            L3[i]=arr3[l+i];
        }
        for (int j=0; j<n2; ++j)
        {
            R[j] = arr[m + 1+ j];
            R1[j]=arr1[m+1+j];
            R2[j]=arr2[m+1+j];
            R3[j]=arr3[m+1+j];
        }
        int i = 0, j = 0;
        int k = l;
        while (i < n1 && j < n2)
        {
            if (L[i] <= R[j])
            {
                arr[k] = L[i];
                arr1[k]=L1[i];
                arr2[k]=L2[i];
                arr3[k]=L3[i];
                i++;
            }
            else
            {
                arr[k] = R[j];
                arr1[k]=R1[j];
                arr2[k]=R2[j];
                arr3[k]=R3[j];
                j++;
            }
            k++;
        }
        while (i < n1)
        {
            arr[k] = L[i];
            arr1[k]=L1[i];
            arr2[k]=L2[i];
            arr3[k]=L3[i];
            i++;
            k++;
        }
        while (j < n2)
        {
            arr[k] = R[j];
            arr1[k]=R1[j];
            arr2[k]=R2[j];
            arr3[k]=R3[j];
            j++;
            k++;
        }
    }
    void sort4(int arr[],int arr1[],int arr2[],int arr3[], int l, int r)
    {
        if (l < r)
        {
            int m = (l+r)/2;
            sort4(arr,arr1,arr2,arr3, l, m);
            sort4(arr ,arr1,arr2,arr3, m+1, r);
            merge4(arr,arr1,arr2,arr3,l, m, r);
        }
    }
    public int justsmall(int a[],int l,int r,int x)
    {
        int p=-1;
        while(l<=r)
        {
            int mid=(l+r)/2;
            if(a[mid]<=x)
            {
                p=mid;
                l=mid+1;
            }
            else
                r=mid-1;
        }
        return p;
    }
    public int justlarge(int a[],int l,int r,int x)
    {
        int p=0;
        while(l<=r)
        {
            int mid=(l+r)/2;
            if(a[mid]<=x)
            {
                l=mid+1;
            }
            else
            {
                p=a[mid];
                r = mid - 1;
            }
        }
        return p;
    }
    long gcd(long x,long y)
    {
        if(x%y==0)
            return y;
        else
            return(gcd(y,x%y));
    }
    long fact(long n)
    {
        long ans=1;
        for(int i=1;i<=n;i++)
            ans*=i;
        return ans;
    }
    public static void main(String args[])throws Exception
    {
        new Thread(null,new templ(),"templ",1<<26).start();
    }
    public void run() {
        try
        {
            InputReader in = new InputReader(System.in);
            PrintWriter out = new PrintWriter(System.out);
            templ ob=new templ();
            int n=in.ni();
            int deg[]=new int[n];
            int xs[]=new int[n];
            Queue<Integer> q=new LinkedList<>();
            for(int i=0;i<n;i++)
            {
                deg[i]=in.ni();
                if(deg[i]==1)
                    q.add(i);
                xs[i]=in.ni();
            }
            int from[]=new int[1000000];
            int to[]=new int[1000000];
            int o=0;
            while(!q.isEmpty())
            {
                int fr=q.remove();
                if(deg[fr]!=1)
                    continue;
                from[o]=fr;
                int t=xs[fr];
                to[o]=t;
                deg[fr]--;
                deg[t]--;
                xs[t]=xs[t]^fr;
                if(deg[t]==1)
                    q.add(t);
                o++;
            }
            out.println(o);
            for(int i=0;i<o;i++)
            {
                out.println(from[i]+" "+to[i]);
            }
            out.close();
        }
        catch(Exception e){
            return;
        }
    }
    static class InputReader {

        private final InputStream stream;
        private final byte[] buf = new byte[8192];
        private int curChar, snumChars;

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

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

        public int ni() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nl() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public int[] nextIntArray(int n) {
            int a[] = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = ni();
            }
            return a;
        }

        public String readString() {
            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 String nextLine() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isEndOfLine(c));
            return res.toString();
        }

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

        private boolean isEndOfLine(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }

    }
}