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 pen(long l, int d){
    if(d>l){
        System.out.print("-1");
        return;
    }
    int turn = 1;
    int left = d-2;
    
    if(left>0 && d<=l){
        for(int i=0; i<l-left; i++){
            if(turn%2 == 0){
                System.out.print('b');
                turn ++;
            }
            else{
                System.out.print('a');
                turn ++;
        }
        }
        switch(left){
        case 1 : System.out.print("c");break;
        case 2 : System.out.print("cd");break;
        case 3 : System.out.print("cde");break;
        case 4 : System.out.print("cdef");break;
        case 5 : System.out.print("cdefg");break;
        case 6 : System.out.print("cdefgh");break;
        case 7 : System.out.print("cdefghi");break;
        case 8 :System.out.print("cdefghij");break;
        case 9 : System.out.print("cdefghijk");break;
        case 10 :System.out.print("cdefghijkl");break;
        case 11 : System.out.print("cdefghijklm");break;
        case 12 :System.out.print("cdefghijklmn"); break;
        case 13 :System.out.print("cdefghijklmno"); break;
        case 14 :System.out.print("cdefghijklmnop"); break;
        case 15 : System.out.print("cdefghijklmnopq"); break;
        case 16 : System.out.print("cdefghijklmnopqr"); break;
        case 17 : System.out.print("cdefghijklmnopqrs");break;
        case 18 : System.out.print("cdefghijklmnopqrst"); break;
        case 19 : System.out.print("cdefghijklmnopqrstu"); break;
        case 20 : System.out.print("cdefghijklmnopqrstuv"); break;
        case 21 : System.out.print("cdefghijklmnopqrstuvw"); break;
        case 22 : System.out.print("cdefghijklmnopqrstuvwx"); break;
        case 23 : System.out.print("cdefghijklmnopqrstuvwxy"); break;
        case 24 : System.out.print("cdefghijklmnopqrstuvwxyz"); break;
        }
        }else{
            if(left == 0){
                for(int i=0; i<l; i++){
                    if(turn%2 == 0){
                        System.out.print('b');
                        turn ++;
                    }
                    else{
                        System.out.print('a');
                        turn ++;
                }
                }
            }else{
                if (l == d)
                    System.out.print("a");
                else 
                    System.out.print("-1");
                }
            }
    
            
        }

public static void main(String[] args){
    Scanner sc =new Scanner (System.in);
    String s = sc.nextLine();
    String[] split = s.split(" ");
    pen(Integer.parseInt(split[0]), Integer.parseInt(split[1]));
    
}
}

