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.IOException;
import java.io.InputStreamReader;


public class C_2013_Apr_2_Div2 {
  public static void main(String[] args) throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String[] split = reader.readLine().split("\\s+");
    int n = Integer.parseInt(split[0]);
    int k = Integer.parseInt(split[1]);
    if (n < k) {
      System.out.println("-1");
      return;
    }
    if (n == 1) {
      System.out.println("a");
      return;
    }
    if (k == 1) {
      System.out.println("-1");
      return;
    }
    char c = 'a';
    for(int i = 0 ; i < n -k + 2; i++) {
      System.out.print(c);
      c = c == 'a' ? 'b': 'a';
    }
    c = 'b';
    for(int i = 0 ; i < k - 2; i++) {
      System.out.print(++c);
    }
    System.out.println("");
  }
}
