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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main
{

    private static BufferedReader reader = 
        new BufferedReader(new InputStreamReader(System.in));

    public static void main(String[] args)
    {
        try
        {
            String[] nos = reader.readLine().split(" ");
            int n = Integer.parseInt(nos[0]);
            int k = Integer.parseInt(nos[1]);
            if(n<k) {
                System.out.println("-1");
            } else if(k == 1) {
                if(n > 1) System.out.println("-1");
                else System.out.println("a");
            } else if(k == 2) {
                for(int i = 1; i<=n; i++) {
                    boolean flag = ((i & 1) == 1);
                    char ch = (char)((!flag) ? 98 : 97);
                    System.out.print(ch);
                }
            } else {
                int last = k - 2;
                for(int i = 1; i<=(n-last); i++) {
                    boolean flag = ((i & 1) == 1);
                    char ch = (char)((!flag) ? 98 : 97);
                    System.out.print(ch);
                }
                for(int j = 1; j<=last; j++) {
                    System.out.print((char)(98+j));
                }
            }
        }
        catch (NumberFormatException e)
        {
            e.printStackTrace();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }
}
