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.util.*;

public class C289 {
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int k = in.nextInt();
        if (k > n || k == 1 && k != n) {
            System.out.println(-1);
        } else if (k == n) {
            for (int i = 0; i < n; i++) {
                System.out.print((char)(i + 'a'));
            }
            System.out.println();
        } else {
            int rem = k - 2;
            char prev = 'b';
            for (int i = 0; i < (n - rem); i++) {
                if (prev == 'b') {
                    System.out.print("a");
                    prev = 'a';
                } else {
                    System.out.print("b");
                    prev = 'b';
                }
            }
            char start = 'c';
            for (int i = 1; i <= rem; i++)
                System.out.print(start++);
            System.out.println();
        }
    }
}
