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.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.StreamTokenizer;

public class C {

    static StreamTokenizer st;
    static PrintWriter pw;

    private static int nextInt() throws IOException {
        st.nextToken();
        return (int) st.nval;
    }

    public static void main(String[] args) throws IOException {
        st = new StreamTokenizer(new BufferedReader(new InputStreamReader(
                System.in)));
        pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
                System.out)));
        // -----------begin-------------------------
        int n = nextInt();
        int k = nextInt();
        if (k > n) {
            pw.print("-1");
            pw.close();
            return;
        }
        if (k == 1 && n > 1) {
            pw.print("-1");
            pw.close();
            return;
        }
        if (k == 1 && n == 1) {
            pw.print("a");
            pw.close();
            return;
        }
        for (int i = 0; i < (n - k) / 2; i++) {
            pw.print("ab");
        }
        if ((n - k) % 2 == 1) {
            pw.print("a");
        }
        if ((n - k) % 2 == 1) {
            pw.print("ba");
            for (int i = 99; i < 99 +  k-2; i++) {
                pw.print((char) (i));
            }
        } else {
            for (int i = 97; i < 97 +  k; i++) {
                pw.print((char) (i));
            }
        }
        pw.close();
        // ------------end--------------------------
    }

}