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.*;
import java.util.*;

public class Timus {
 public static void main(String[] args) throws IOException {
  BufferedReader rd = new BufferedReader(new InputStreamReader(System.in));
  // PrintWriter pw = new PrintWriter(System.out);
  // int n = Integer.parseInt(rd.readLine());
  StringTokenizer st = new StringTokenizer(rd.readLine());
  int n = Integer.parseInt(st.nextToken());
  int k = Integer.parseInt(st.nextToken());
  if (n == 1 && k == 1)
   System.out.println("a");
  else {
   if (k > n || (n > 1 && k == 1))
    System.out.println("-1");
   else {
    for (int i = 0; i < n - k + 2; i++) {
     if (i % 2 == 0)
      System.out.print("a");
     else
      System.out.print("b");
    }
    for (int i = 0; i < k - 2; i++)
     System.out.print((char) ('a' + i + 2));
   }
  }
 }
}