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.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;



public class CmdProgram {
    protected StringTokenizer tokenizer;
    protected PrintWriter outputWriter;
    
    CmdProgram(){
        BufferedReader inputReader = new BufferedReader(new InputStreamReader(System.in));
        this.outputWriter = new PrintWriter(System.out);
        try{
            this.tokenizer = new StringTokenizer(inputReader.readLine());
        }catch(Exception e){
            this.outputWriter.printf("-1");
            this.outputWriter.close();
        }finally{
            try{inputReader.close();}catch(Exception e){};
        }
    }
    
    
    public String nextInput(){
        return this.tokenizer.nextToken();
    }
    
    public int nextInt(){
        return Integer.parseInt(nextInput());
    }
    
    public static void main(String[] args){
        PenguinPoloString polo = new PenguinPoloString();
        int n = polo.nextInt();
        int k = polo.nextInt();
        String result = polo.makeString(n, k);
        polo.outputWriter.write(result);
        polo.outputWriter.close();
    }
}


class PenguinPoloString extends CmdProgram{

    public String makeString(int n, int k){
        StringBuffer str = new StringBuffer();

        if(k==1){
            if(n>1){
                str.append("-1");
            }else{
                str.append("a");
            }
        }else{
            if(n < k){
                str.append("-1");
            }else{
                int dif = n-k;
                for(int i=0; i< dif/2+1; i++){
                    str.append("ab");
                }
                for(int i=0; i< dif%2; i++){
                    str.append("a");
                }
                for(int i=0; i< k-2; i++){
                    str.append((char)((int)'a'+i+2));
                }
            }
        }
        return str.toString();

    }
    
}
