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.IOException;
import java.util.Scanner;

public class CodeForces_PolothePenguinAndStrings {
    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(System.in);
        while (in.hasNext()) {

            System.out.println(getAns(in.nextInt(), in.nextInt()));
        }
    }

    private static String getAns(int n, int k) {

        if (n < k || (k == 1 && n > 1))
            return "-1";

        char c = 'a';
        boolean b = true;
        boolean aa = true, bb = true;
        StringBuilder s = new StringBuilder();

        if (n == k) {
            for (int i = 0; i < n; i++) {
                s.append(c++);
            }
            return s.toString();
        }

        for (int i = 0; i < n; i++) {
            if (n - i > k) {
                if (b) {
                    c = 'a';
                    if (aa) {
                        aa = false;
                        k--;
                    }
                } else {
                    c = 'b';
                    if (bb) {
                        bb = false;
                        k--;
                    }
                }
                b = !b;
                s.append(c);
            } else {
                if (c == 'a')
                    c++;

                c++;
                s.append(c);
                k--;
            }

        }
        return s.toString();
    }
}
