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.*;
import java.security.spec.ECField;
import java.util.HashMap;
import java.util.StringTokenizer;



public class Main {

    static int a[];
    static int l[];
    static int ans[];
    static HashMap<Integer, Boolean> map = new HashMap<>();

    public static void main (String [] args) throws Exception {
        InputReader reader = new InputReader();
        PrintWriter out = new PrintWriter(System.out);

        int n, k;
        n = reader.nextInt();
        k = reader.nextInt();
        int [] str = new int[n];

        if(n<k){
            out.println("-1");
            out.flush();

            return;
        }
        if(k==1){
            if(n>1)
                out.println("-1");
            else
               out.println("a");

            out.flush();
            return;
        }

        //char c= 'a'; 97


        for (int i =0 ; i < (n-k+2) ; i ++)
        {
            if(i<n) {
                if (i % 2 == 0) str[i] = 0;
                else str[i] = 1;
            }
        }
        int j=2;
        for (int i =n-k+2 ; i < n ; i ++)
        {
            str[i]=j++;
        }



        for (int i =0 ; i < n ; i ++)
        {
            out.print( (char) (str[i]+97)  );
        }



        out.flush();
    }


    static class InputReader {
        BufferedReader reader;
        StringTokenizer tokenizer;

        InputReader() {
            reader = new BufferedReader(new InputStreamReader(System.in));
            tokenizer = null;
        }

        String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return tokenizer.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }
        double nextDouble() {
            return Double.parseDouble(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }
    }
}
