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.util.*;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
/**
 *
 * @author N-AssassiN
 */
public class Main {

    private static BufferedReader reader;
    private static BufferedWriter out;
    private static StringTokenizer tokenizer;
    //private final static String filename = "filename";

    /**
     * call this method to initialize reader for InputStream and OututStream
     */
    private static void init(InputStream input, OutputStream output) {
        reader = new BufferedReader(new InputStreamReader(input));
        out = new BufferedWriter(new OutputStreamWriter(output));
        //reader = new BufferedReader(new FileReader(filename + ".in"));
        //out = new BufferedWriter(new FileWriter(filename + ".out"));
        tokenizer = new StringTokenizer("");
    }

    /**
     * get next word
     */
    private static String next() throws IOException {
        while (!tokenizer.hasMoreTokens()) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(reader.readLine());
        }
        return tokenizer.nextToken();
    }

    private static int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    private static long nextLong() throws IOException {
        return Long.parseLong(next());
    }

    private static double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    public static void main(String[] args) throws IOException {
        init(System.in, System.out);
        int n = nextInt();
        int k = nextInt();
        if (n < k) {
            System.out.println("-1");
        } else if (k == 1) {
            if (n == 1) {
                out.write("a\n");
            } else {
                out.write("-1\n");
            }
            out.flush();
        } else {
            int flag = n - k + 2;
            boolean odd = false;
            for (int i = 0; i < flag; i++) {
                if (odd) {
                    out.write("b");
                    odd = false;
                } else {
                    out.write("a");
                    odd = true;
                }
            }
            int count = 'c';
            for (int i = flag; i < n; i++) {
                out.write(((char) (count++)) + "");
            }
            out.write("\n");
            out.flush();
        }
    }
}