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.Scanner;

public class PoloStrings implements Runnable {
  @Override
  public void run() {
    try {
      Scanner in = new Scanner(System.in);
      int n = in.nextInt();
      int k = in.nextInt();

      if (n == 1 && k == 1) {
        System.out.println("a");
        return;
      }

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

      char[] s = new char[n];

      if (k == n - 1) {
        s[0] = 'a';
        s[1] = 'b';
        s[2] = 'a';
        for (int i = 3; i < n; i++) {
          s[i] = (char)('c' + (i - 3));
        }
      } else {
        for (int i = 0; i < n - k + 2; i++) {
          if (i % 2 == 0) s[i] = 'a';
          else s[i] = 'b';
        }
        for (int i = n - k + 2; i < n; i++)
          s[i] = (char)('c' + (i - n + k - 2));
      }

      System.out.println(new String(s));
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }

  public static void main(String[] args) {
    new Thread(new PoloStrings()).start();
  }
}
