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.math.BigInteger;
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Codeforces2 implements Runnable {

    private BufferedReader br = null;
    private PrintWriter pw = null;
    private StringTokenizer stk = new StringTokenizer("");

    public static void main(String[] args) {
        new Thread(new Codeforces2()).run();
    }

    public void run() { /*
                         * try { // br = new BufferedReader(new FileReader("input.txt")); pw = new PrintWriter("output.txt"); } catch (FileNotFoundException e)
                         * { e.printStackTrace(); }
                         */
        br = new BufferedReader(new InputStreamReader(System.in));
        pw = new PrintWriter(new OutputStreamWriter(System.out));
        solver();
        pw.close();

    }

    private void nline() {
        try {
            if (!stk.hasMoreTokens())
                stk = new StringTokenizer(br.readLine());
        } catch (IOException e) {
            throw new RuntimeException("KaVaBUnGO!!!", e);
        }
    }

    private String nstr() {
        while (!stk.hasMoreTokens())
            nline();
        return stk.nextToken();
    }

    private int ni() {
        return Integer.valueOf(nstr());
    }

    private long nl() {
        return Long.valueOf(nstr());
    }

    private double nd() {
        return Double.valueOf(nstr());
    }

    String nextLine() {
        try {
            return br.readLine();
        } catch (IOException e) {
        }
        return null;
    }

    private void solver() {
        String alf = "cdefghijklmnopqrstuvwxyz";
        int n = ni(), k = ni();
        StringBuffer sb = new StringBuffer();
        if (n == 1 && k == 1) {
            System.out.println("a");
        } else if (n < k || k <= 1) {
            System.out.println(-1);
        } else {
            for (int i = 0; i < (n - k + 2); i++) {
                if (i % 2 == 0) {
                    sb.append('a');
                } else {
                    sb.append('b');
                }
            }
            sb.append(alf.substring(0, k - 2));
            System.out.println(sb.toString());
        }

    }

    private BigInteger nbi() {
        return new BigInteger(nstr());
    }

    void exit() {
        System.exit(0);
    }

}
