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;

public class PoloStrings {
    private static char[] alphabets = {'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'};
    private static char[] odd = {'b', 'a', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
    
    public static void main (String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int k = scanner.nextInt();

        System.out.println(PoloStrings.getString(n, k)); 
    }

    public static String getString(int n, int k) {
        int diff = n - k;
        if(diff < 0 || (k == 1 && n > 1)) {
            return "-1";
        } 

        StringBuffer buffer = new StringBuffer();

        int i = 0;
        while (diff != 0) {
            buffer.append(alphabets[i++ % 2]);
            diff--;
        }

        if (i++ % 2 == 1) {
            i = 0;
            while(k > 0) {
                buffer.append(odd[i++]);
                k--;
            }
        } else {
            i = 0;
            while(k > 0) {
                buffer.append(alphabets[i++]);
                k--;
            }
        }

        
        

        return buffer.toString();
    }
}