Problem: There are n cities in Berland, each of them has a unique id — an integer from 1 to n, the capital is the one with id 1. Now there is a serious problem in Berland with roads — there are no roads.

That is why there was a decision to build n - 1 roads so that there will be exactly one simple path between each pair of cities.

In the construction plan t integers a1, a2, ..., at were stated, where t equals to the distance from the capital to the most distant city, concerning new roads. ai equals the number of cities which should be at the distance i from the capital. The distance between two cities is the number of roads one has to pass on the way from one city to another. 

Also, it was decided that among all the cities except the capital there should be exactly k cities with exactly one road going from each of them. Such cities are dead-ends and can't be economically attractive. In calculation of these cities the capital is not taken into consideration regardless of the number of roads from it. 

Your task is to offer a plan of road's construction which satisfies all the described conditions or to inform that it is impossible.

Input

The first line contains three positive numbers n, t and k (2 ≤ n ≤ 2·105, 1 ≤ t, k < n) — the distance to the most distant city from the capital and the number of cities which should be dead-ends (the capital in this number is not taken into consideration). 

The second line contains a sequence of t integers a1, a2, ..., at (1 ≤ ai < n), the i-th number is the number of cities which should be at the distance i from the capital. It is guaranteed that the sum of all the values ai equals n - 1.

Output

If it is impossible to built roads which satisfy all conditions, print -1.

Otherwise, in the first line print one integer n — the number of cities in Berland. In the each of the next n - 1 line print two integers — the ids of cities that are connected by a road. Each road should be printed exactly once. You can print the roads and the cities connected by a road in any order.

If there are multiple answers, print any of them. Remember that the capital has id 1.

Examples

Input

7 3 3
2 3 1


Output

7
1 3
2 1
2 6
2 4
7 4
3 5


Input

14 5 6
4 4 2 2 1


Output

14
3 1
1 4
11 6
1 2
10 13
6 10
10 12
14 12
8 4
5 1
3 7
2 6
5 9


Input

3 1 1
2


Output

-1

Solution (in JAVA):

Solution:


import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.*;


public class G{
	static long mod=1000000007;
	
    public static void main(String[] args) throws Exception{           	
    	InputReader in = new InputReader(System.in);       
    	PrintWriter pw=new PrintWriter(System.out);   
    	
    	
    	int n=in.readInt();
        int t=in.readInt();
        int k=in.readInt();
        int a[]=new int[t];
        int f=1;
        
    	for(int i=0;i<t;i++)
    	{
    		a[i]=in.readInt();
    		
    	}
    	StringBuilder sb=new StringBuilder("");
    	sb.append(n).append("\n");
    	int c=1;
    	ArrayList<Integer>ar[]=new ArrayList[t+1];
    	TreeSet<Integer>hs[]=new TreeSet[t+1]; //leaf
    	for(int i=0;i<=t;i++)
    		{
    			ar[i]=new ArrayList<Integer>();
    			hs[i]=new TreeSet<Integer>();
    		}
    	ar[0].add(1);
    	
    	
    	for(int i=1;i<=t;i++)
    	{
    		c+=1;
    		sb.append(i+" "+(i+1)).append("\n");;
    		ar[i].add(i+1);
    	}
    	int req=n-k-t;
    	if(req<0)
    	{
    		System.out.println(-1);
    		System.exit(0);
    	}
    	for(int i=1;i<a[0];i++)
    	{
    		c+=1;
    		hs[1].add(c);
    		sb.append(1+" "+(c)).append("\n");;
    	}
    //	System.out.println(req);
    	for(int i=1;i<t;i++)
    	{
    		int x=a[i];
    		for(int j=0;j<x-1;j++)
    		{
    			c++;
    			if(req>0)
    			{
    				if(hs[i].size()>0)
    				{
    					int y=hs[i].first();
    					hs[i].remove(y);
    					sb.append(y+" "+c).append("\n");;
    					hs[i+1].add(c);
    					req--;
    				}
    				else
        			{
        				int y=ar[i].get(0);
        				sb.append(y+" "+c).append("\n");
    					hs[i+1].add(c);
    					//req--;
        			}
    			}
    			else
    			{
    				int y=ar[i].get(0);
    				sb.append(y+" "+c).append("\n");
					hs[i+1].add(c);
					//req--;
    			}
    		}
    	}
    	if(req>0)
    	{
    		//System.out.println(req);
    		System.out.println(-1);
    		System.exit(0);
    	}
    	System.out.println(sb);
       
    	pw.close();
    }
    

	
public static long gcd(long x,long y)
{
	if(x%y==0)
		return y;
	else
		return gcd(y,x%y);
}
public static int gcd(int x,int y)
{
	if(x%y==0)
		return y;
	else 
		return gcd(y,x%y);
}
public static int abs(int a,int b)
{
	return (int)Math.abs(a-b);
}
public static long abs(long a,long b)
{
	return (long)Math.abs(a-b);
}
public static int max(int a,int b)
{
	if(a>b)
		return a;
	else
		return b;
}
public static int min(int a,int b)
{
	if(a>b)
		return b;
	else 
		return a;
}
public static long max(long a,long b)
{
	if(a>b)
		return a;
	else
		return b;
}
public static long min(long a,long b)
{
	if(a>b)
		return b;
	else 
		return a;
}


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 long pow(long n,long p)
{
	long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;	    
	    p >>=1;
	    n*=n;	    
	}
	return result;

}
static class Pair implements Comparable<Pair>
{
	int a,b;
	Pair (int a,int b)
	{
		this.a=a;
		this.b=b;
	}

	public int compareTo(Pair o) {
		// TODO Auto-generated method stub
		if(this.a!=o.a)
		return Integer.compare(this.a,o.a);
		else
			return Integer.compare(this.b, o.b);
		//return 0;
	}
	public boolean equals(Object o) {
        if (o instanceof Pair) {
            Pair p = (Pair)o;
            return p.a == a && p.b == b;
        }
        return false;
    }
    public int hashCode() {
        return new Integer(a).hashCode() * 31 + new Integer(b).hashCode();
    }
    
} 
    
static long sort(int a[])
{  int n=a.length;
	int b[]=new int[n];	
	return mergeSort(a,b,0,n-1);}
static long mergeSort(int a[],int b[],long left,long right)
{ long c=0;if(left<right)
 {   long mid=left+(right-left)/2;
	 c= mergeSort(a,b,left,mid);
	 c+=mergeSort(a,b,mid+1,right);
	 c+=merge(a,b,left,mid+1,right); }	
	return c;	 }
static long merge(int a[],int  b[],long left,long mid,long right)
{long c=0;int i=(int)left;int j=(int)mid; int k=(int)left;
while(i<=(int)mid-1&&j<=(int)right)
{ if(a[i]<=a[j]) {b[k++]=a[i++]; }
else	{ b[k++]=a[j++];c+=mid-i;}}
while (i <= (int)mid - 1)   b[k++] = a[i++]; 
while (j <= (int)right) b[k++] = a[j++];
for (i=(int)left; i <= (int)right; i++) 
	a[i] = b[i];  return c;  }
    
public static int[] radixSort(int[] f)
{
	int[] to = new int[f.length];
	{
		int[] b = new int[65537];
		for(int i = 0;i < f.length;i++)b[1+(f[i]&0xffff)]++;
		for(int i = 1;i <= 65536;i++)b[i]+=b[i-1];
		for(int i = 0;i < f.length;i++)to[b[f[i]&0xffff]++] = f[i];
		int[] d = f; f = to;to = d;
	}
	{
		int[] b = new int[65537];
		for(int i = 0;i < f.length;i++)b[1+(f[i]>>>16)]++;
		for(int i = 1;i <= 65536;i++)b[i]+=b[i-1];
		for(int i = 0;i < f.length;i++)to[b[f[i]>>>16]++] = f[i];
		int[] d = f; f = to;to = d;
	}
	return f;
}    
static  class InputReader
{
    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public InputReader(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 readInt()
    {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-')
        {
            sgn = -1;
            c = read();
        }
        int 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 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 readLine() {
        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 double readDouble() {
        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, readInt());
            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, readInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                m /= 10;
                res += (c - '0') * m;
                c = read();
            }
        }
        return res * sgn;
    }
    public long readLong() {
        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 boolean isSpaceChar(int c)
    {
        if (filter != null)
            return filter.isSpaceChar(c);
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public String next()
    {
        return readString();
    }

    public interface SpaceCharFilter
    {
        public boolean isSpaceChar(int ch);
    }
    public boolean isEndOfLine(int c) {
        return c == '\n' || c == '\r' || c == -1;
    }
}

  
//BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
	//StringBuilder sb=new StringBuilder("");
	 //InputReader in = new InputReader(System.in);
	//PrintWriter pw=new PrintWriter(System.out);
	//String line=br.readLine().trim();
	       	
	//int t=Integer.parseInt(br.readLine());
  //	while(t-->0)
  	//{
  	//int n=Integer.parseInt(br.readLine());
	//long n=Long.parseLong(br.readLine());
	//String l[]=br.readLine().split(" ");
  //int m=Integer.parseInt(l[0]);
	//int k=Integer.parseInt(l[1]);
	//String l[]=br.readLine().split(" ");
	//l=br.readLine().split(" ");
	/*int a[]=new int[n];
	for(int i=0;i<n;i++)
	{
		a[i]=Integer.parseInt(l[i]);
	}*/
	   //System.out.println(" ");	     	
	
	//}
}