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
//package topcoders;

import java.util.Scanner;

public class C2apr3 {

    /**
     * @param args
     */
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int dist = sc.nextInt();
        int[] a = new int[26];
        String str = new String();
        
        for (int i = 0; i< 26; i++) {
            a[i] = i + 97;
        }

        if (dist > n) {
            System.out.print("-1");
            return;
        }

       
        if (n < 2) {
            str = (n == 1) ? "a" : "";
            System.out.print(str);
            return;
        }
        
        if (dist < 2) {
            System.out.print("-1");
            return;
        }
        
        String inter = createInterString(n-dist);

        for (int i = 0; i < dist; i++) {
            str += (char) a[i];
        }

        String finStr = str.substring(0, 2);
        finStr += inter;
        finStr += str.substring(2);

        System.out.print(finStr);

    }
    
    

    private static String createInterString(int length) {
        
        String str = new String();
        
        if (length == 0)
            return "";
        
        if (length == 1)
            return "a";
        
        else
            str = "ab";
            
        while (str.length() <= (length/2)) {
            str +=str;
        }
        
        str +=str;
        
        return str.substring(0, length);
    }
        

}
