One day, at the "Russian Code Cup" event it was decided to play football as an out of competition event. All participants was divided into n teams and played several matches, two teams could not play against each other more than once.

The appointed Judge was the most experienced member — Pavel. But since he was the wisest of all, he soon got bored of the game and fell asleep. Waking up, he discovered that the tournament is over and the teams want to know the results of all the matches.

Pavel didn't want anyone to discover about him sleeping and not keeping an eye on the results, so he decided to recover the results of all games. To do this, he asked all the teams and learned that the real winner was friendship, that is, each team beat the other teams exactly k times. Help Pavel come up with chronology of the tournir that meets all the conditions, or otherwise report that there is no such table.

Input

The first line contains two integers — n and k (1 ≤ n, k ≤ 1000).

Output

In the first line print an integer m — number of the played games. The following m lines should contain the information about all the matches, one match per line. The i-th line should contain two integers ai and bi (1 ≤ ai, bi ≤ n; ai ≠ bi). The numbers ai and bi mean, that in the i-th match the team with number ai won against the team with number bi. You can assume, that the teams are numbered from 1 to n.

If a tournir that meets the conditions of the problem does not exist, then print -1.

Examples

Input

3 1


Output

3
1 2
2 3
3 1
import java.util.* ;
import java.io.* ;
import java.util.regex.* ;
import java.text.* ;
import java.math.* ;

public class Main {
	public static void main( String[] args ) {
		InputStream isObj ;
		OutputStream osObj ;
		InputReader irObj ;
		PrintWriter pwObj ;
		String inputFileName , outputFileName ;
		File fObj1 , fObj2 ;
		int T , ind ;
		boolean hasMoreInput , hasTestCases ;
		ProblemSolver psObj ;
		//only variable to be changed here is hasTestCases and osObj(optional for file output)
		hasTestCases = ProblemSolver.HAS_TEST_CASES ;
		isObj = System.in ;
		osObj = System.out ;
		inputFileName = ProblemSolver.INPUT_FILE_NAME ;
		outputFileName = ProblemSolver.OUTPUT_FILE_NAME ;
		try {
			fObj1 = new File( System.getProperty( "user.dir" ).toString() + "\\" + inputFileName ) ;
			fObj2 = new File( System.getProperty( "user.dir" ).toString() + "\\" + outputFileName ) ;
			if( fObj1.exists() == false ) {
				fObj1 = new File( System.getProperty( "user.dir" ).toString() + "\\src\\" + inputFileName ) ;
				fObj2 = new File( System.getProperty( "user.dir" ).toString() + "\\src\\" + outputFileName ) ;
			}
			isObj = new FileInputStream( fObj1 ) ;
			if( ProblemSolver.DO_OUTPUT_TO_FILE == true ) {
				osObj = new FileOutputStream( fObj2 ) ;
			}
		}
		catch( Exception ex ) {
			ex.printStackTrace() ;
			isObj = System.in ;
			osObj = System.out ;
		}
		irObj = new InputReader( isObj ) ;
		pwObj = new PrintWriter( osObj ) ;
		try {
			psObj = new ProblemSolver() ;
			if( hasTestCases == true ) {
				//for input with test cases
				T = irObj.nextInt() ;
				for( ind = 1 ; ind <= T ; ind++ ) {        	
					psObj.clearPerCase() ;
					psObj.getInput( irObj ) ;
					psObj.solveCase( pwObj ) ;
				}
			}
			else {
				//for end of file input
				for( ind = 1 ; ; ind++ ) {        	
					psObj.clearPerCase() ;
					hasMoreInput = psObj.getInput( irObj ) ;
					if( hasMoreInput == false ) {
						break ;
					}
					psObj.solveCase( pwObj ) ;
				}
			}
			pwObj.flush() ;
			isObj.close() ;
			osObj.close() ;
		}
		catch( Exception ex ) {
			ex.printStackTrace() ;
			throw new RuntimeException( ex ) ;
		}
		finally {
			pwObj.close() ;
		}
	}
}

class ProblemSolver {
	public static final boolean HAS_TEST_CASES = false ;
	public static final String INPUT_FILE_NAME = "test.in" ;
	public static final String OUTPUT_FILE_NAME = "out.txt" ;
	public static final boolean DO_OUTPUT_TO_FILE = false ;
	
	public void solveCase( PrintWriter pwObj ) {
		int i , j , a , b , c ;
		int res ;
		res = 0 ;
		this.cn = 0 ;
		for( i = 1 ; i <= this.n ; i++ ) {
			a = 0 ;
			for( j = 1 ; j <= this.n ; j++ ) {
				if( i != j && this.done[ i ][ j ] == 1 ) {
					a++ ;
				}
			}
			if( a > this.k ) {
				res = -1 ;
			}
			if( a == this.k ) {
				continue ;
			}
			for( j = 1 ; j <= this.n ; j++ ) {
				if( i != j && this.done[ i ][ j ] == 0 && this.done[ j ][ i ] == 0 ) {
					this.done[ i ][ j ] = 1 ;
					this.done[ j ][ i ] = -1 ;
					this.arr[ this.cn ] = i ;
					this.brr[ this.cn ] = j ;
					this.cn++ ;
					a++ ;
					if( a >= this.k ) {
						break ;
					}
				}
			}
			if( a != this.k ) {
				res = -1 ;
			}
		}
		if( res == -1 ) {
			pwObj.println( res ) ;
		}
		else {
			pwObj.println( this.cn ) ;
			for( i = 0 ; i < this.cn ; i++ ) {
				pwObj.println( this.arr[ i ] + " " + this.brr[ i ] ) ;
			}
		}
	}
	
	private void init() {
		//default values
		this.lim1 = 1000010 ;
		this.lim2 = 1010 ;
		this.cc = 1 ;
		this.ind = 0 ;
		this.declareArrays() ;
		this.fillAllArrays() ;
	}
	
	public boolean getInput( InputReader irObj ) {
		int i ;
		try {
			this.n = irObj.nextInt() ;
		}
		catch( Exception ex ) {
			return false ;
		}
		this.k = irObj.nextInt() ;
		return true ;
	}
	
	private int lim1 , lim2 , cn , ind , cc , n , k ; 
	private String[] srr ;
	private int[] arr , brr ;
	private Node[] nrr ;
	private long[] lrr ;
	private int[][] memo , done ;
	private ArrayList< Integer >[] adj_list ;
	
	public void clearPerCase() {
		this.incrementInd() ;
		this.incrementCc() ;
		this.n = 0 ;
		this.cn = 0 ;
		for( int zrr[] : this.done ) {
			Arrays.fill( zrr , 0 ) ;
		}
	}
	
	private void declareArrays() {
		//array declarations
		this.arr = new int[ this.lim1 ] ;
		this.brr = new int[ this.lim1 ] ;
		this.lrr = new long[ this.lim1 ] ;
		this.srr = new String[ this.lim1 ] ;
		this.nrr = new Node[ this.lim1 ] ;
		this.memo = new int[ this.lim2 ][ this.lim2 ] ;
		this.done = new int[ this.lim2 ][ this.lim2 ] ;
		this.adj_list = new ArrayList[ this.lim1 ] ;
	}
	
	private void fillAllArrays() {
		int i ;
		//array initiations
		Arrays.fill( this.arr , 0 ) ;
		Arrays.fill( this.brr , 0 ) ;
		Arrays.fill( this.lrr , 0 ) ;
		for( int zrr[] : this.memo ) {
			Arrays.fill( zrr , -1 ) ;
		}
		for( int zrr[] : this.done ) {
			Arrays.fill( zrr , 0 ) ;
		}
		for( i = 0 ; i < this.lim1 ; i++ ) {
			this.srr[ i ] = new String( "" ) ;
			this.nrr[ i ] = new Node( Integer.MIN_VALUE , Integer.MIN_VALUE ) ;
			this.adj_list[ i ] = new ArrayList< Integer >() ;
		}
	}
	
	private void incrementInd() {
		this.ind++ ;
	}
	
	public void setCn( int c ) {
		this.cn = c ;  
	}
	
	public void incrementCn() {
		this.cn++ ;
	}
	
	public void incrementCc() {
		this.cc++ ;
	}
	
	public int getCn() {
		return this.cn ;
	}
	
	public ProblemSolver() {
		this.init() ;
	}
}

class Node implements Comparable< Node > {
	private int v , f ;
	
	private Node() {
	}
	
	public Node( int vParam , int fParam ) {
		this.v = vParam ;
		this.f = fParam ;
	}
	
	public void setV( int vParam ) {
		this.v = vParam ;
	}
	
	public void setF( int fParam ) {
		this.f = fParam ;
	}
	
	public int getV() {
		return this.v ;
	}
	
	public int getF() {
		return this.f ;
	}
	
	public int compareTo( Node nParam ) {
		return this.getV() - nParam.getV() ;
	}
}

class InputReader {
	private BufferedReader reader ;
	private StringTokenizer tokenizer ;

	public InputReader( InputStream stream ) {
		this.reader = new BufferedReader( new InputStreamReader( stream ) ) ;
		this.tokenizer = null ;
	}

	private String next( int fl ) {
		while( this.tokenizer == null || ! this.tokenizer.hasMoreTokens() ) {
			try {                
				this.tokenizer = new StringTokenizer( this.reader.readLine() ) ;
			}
			catch( IOException e ) {
				throw new RuntimeException( e ) ;
			}
		}
		String s ;
		s = "" ;
		if( fl == 1 ) {
			s = this.tokenizer.nextToken() ; 
		}
		return s ;
	}

	public String nextLine() {
		this.next( 0 ) ;
		int fl ;
		String line ;
		line = "" ;
		fl = 0 ;
		if( this.tokenizer != null ) {
			while( this.tokenizer.hasMoreTokens() ) {
				if( fl == 1 ) {
					line += " " ;
				}
				fl = 1 ;
				line += this.tokenizer.nextToken() ;
			}
		}
		return line ;
	}

	public int nextInt() {
		return Integer.parseInt( this.next( 1 ) ) ;
	}

	public long nextLong() {
		return Long.parseLong( this.next( 1 ) ) ;
	}

	public double nextDouble() {
		return Double.parseDouble( this.next( 1 ) ) ;
	}

	public String nextString() {
		return this.next( 1 ) ;
	}
}
