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.Scanner;

/**
 *
 * @author Feli
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner input = new Scanner(System.in);
        char[] abc = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
                'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
        int n = input.nextInt();
        int k = input.nextInt();
                StringBuilder sb = new StringBuilder();
        if(k>n || k>26 || (k==1 && n>1)){
            System.out.println(-1);
        }else if(n == 26 && k == 26){
            System.out.println("abcdefghijklmnopqrstuvwxyz");
        }else if(k==1 && n == 1){
            System.out.println("a");
        }else {
            int min = 0;
            for(int i=0;i<n;i++){
                if(i == n-k){
                    
                    if(min == 0 && sb.length()>0){
                        min += 2;
                    }else if(min == 1){
                        min += 1;
                    }
                    for(int j = n-k;j<n;j++){
                        sb.append(abc[min]);
                        min++;
                    }
                }else {
                    if(min == 0){
                        sb.append(abc[min]);
                        min = 1;
                        if(sb.length() == 1){
                            k--;
                        }
                    }else if(min == 1){
                        sb.append(abc[min]);
                        min = 0;
                        if(sb.length() == 2){
                            k--;
                        }
                    }
                }
            }
            System.out.println(sb.toString());
        }

    }

}