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.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.PrintWriter;
import java.util.Scanner;

public class C177 {
    public static void main(String[] args) {
        Scanner in = new Scanner(new BufferedInputStream(System.in));
        PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
        int n = in.nextInt();
        int k = in.nextInt();
        if (n < k) out.print("-1");
        else if (k == 1) out.print(n > 1 ? "-1" : "a");
        else {
            char[] res = new char[n];
            for (int i = 0; i < n - (k - 2); i++) {
                res[i] = i % 2 == 0 ? 'a' : 'b';
            }
            for (int i = n - (k - 2); i < n; i++) {
                res[i] = (char)('a' + i - (n - (k - 2)) + 2);
            }
            for (int i = 0; i < n; i++) {
                out.print(res[i]);
            }
        }
        out.close();
    }
}
