Little penguin Polo adores strings. But most of all he adores strings of length n.

One day he wanted to find a string that meets the following conditions:

  1. The string consists of n lowercase English letters (that is, the string's length equals n), exactly k of these letters are distinct. 
  2. No two neighbouring letters of a string coincide; that is, if we represent a string as s = s1s2... sn, then the following inequality holds, si ≠ si + 1(1 ≤ i < n). 
  3. Among all strings that meet points 1 and 2, the required string is lexicographically smallest. 



Help him find such string or state that such string doesn't exist.

String x = x1x2... xp is lexicographically less than string y = y1y2... yq, if either p < q and x1 = y1, x2 = y2, ... , xp = yp, or there is such number r (r < p, r < q), that x1 = y1, x2 = y2, ... , xr = yr and xr + 1 < yr + 1. The characters of the strings are compared by their ASCII codes.

Input

A single line contains two positive integers n and k (1 ≤ n ≤ 106, 1 ≤ k ≤ 26) — the string's length and the number of distinct letters.

Output

In a single line print the required string. If there isn't such string, print "-1" (without the quotes).

Examples

Input

7 4


Output

ababacd


Input

4 7


Output

-1
import java.util.*;
import java.io.*;
import java.math.*;


public class Main {


   public static void main( String[] args ) throws IOException{
      long inicio = System.currentTimeMillis();
      BufferedReader in = new BufferedReader( new InputStreamReader( System.in ) );
      String[] x = in.readLine().split( " " );
      int n = Integer.parseInt( x[0] ), k = Integer.parseInt( x[1] );
      String sol = "";
      StringBuilder sb = new StringBuilder();
      if( ( k <= 1 && n > 1 ) || ( k > n ) ) sol += -1;
      else if( k > 1 ){
         for( int i = 0; i < n - k + 2; i++ )
            if( i % 2 == 0 ) sb.append( "a" );
            else sb.append( "b" );
         char c = 'c';
         for( int i = 0; i < k - 2; i++ ){
            sb.append( c );
            c++;
         }
         sol += sb.toString();
      }else sol += "a";

      p( sol );
   }
   public static void pln( String s ){
      System.out.println( s );
   }
   public static void p( String s ){
      System.out.print( s );
   }
}
