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.*;
public class Main
{
    public static void main(String[] args){
        Scanner sc  = new Scanner(System.in);
        int tam     = sc.nextInt();
        int lim     = sc.nextInt();
        String abc="";
        if(tam >= lim && lim != 1){
        abc = "cdefghijklmnopqrstuvwxyz";
        String cola = abc.substring(0, (lim-2));
        int cant = (((tam-cola.length())/2));
        String[] ab = {"ab", "abab", "ababab", "abababab", "ababababab", "abababababab", "ababababababab", "abababababababab", "ababababababababab", "abababababababababab"};
        if(cant%10!=0){
            System.out.print(ab[(cant%10)-1]);
        }
        cant /= 10;
        for(int i=0; i<cant; i++)
            System.out.print(ab[9]);  
        System.out.print(((tam-cola.length())%2!=0?"a":"") + cola);
        }
        abc = lim + tam ==2? "a" : abc; 
        System.out.print(lim + tam ==2? "a" : "");
        System.out.println(abc.length()==0?-1:"");
    }
}