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.Scanner;

/**
 * User: Oleksiy Pylypenko
 * Date: 1/6/13
 * Time: 1:46 PM
 */
public class C {
    static boolean debug = !"true".equalsIgnoreCase(System.getProperty("ONLINE_JUDGE"));
    private Scanner in;
    private PrintWriter out;

    private void solve() throws IOException {
        final int n = in.nextInt(), k = in.nextInt();

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

        for (int i = 0; i < n; i++) {
            if (i < n - k + 2) out.print((i % 2==0) ? 'a' : 'b');
            else {
                int letter = i - (n - k + 2);
                out.print((char)('c' + letter));
            }
        }
        out.println();
    }

    public static void main(String[] args) throws IOException {
        new C().exec();
    }

    private void exec() {
        if (debug) {
            try {
                System.setIn(new FileInputStream(getClass().getName() + ".in"));
            } catch (FileNotFoundException e) {
                System.out.println(e);
                System.exit(1);
            }
        }
        in = new Scanner(new InputStreamReader(System.in));
        out = new PrintWriter(System.out, true);

        try {
            if (debug) {
                while (in.hasNext()) {
                    solve();
                }
            } else {
                solve();
            }
        } catch (IOException e) {
            if (debug) {
                System.out.println(e);
                System.exit(1);
            }
        } finally {
            out.close();
            in.close();
        }
    }

}
