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 Main {
    static StringBuilder sb = new StringBuilder();
        public static void main(String[] args) throws IOException {
                BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
                String[] s = bf.readLine().split(" ");
                int n = Integer.parseInt(s[0]);
                int k = Integer.parseInt(s[1]);
                if(k>n||(n > 1 && k == 1)){
                        System.out.println(-1);
                        return;
                }
                char[] alphabets = new char[26];
                if(n == 1 && k == 1){
            sb.append("a");
            
        }else{
 
            int m = 0;
            for (int i = 97; i < 123; i++ , m++) {
                alphabets[m] = ((char) i);
            }
            int total = (n-k)+2;
            boolean getout = true;
            for (int i = 0; i < total; i++) {
                if(getout)
                    sb.append("a");
                else
                    sb.append("b");
                getout = !getout;
 
            }
            for (int i = 0; i < n-total; i++) {
                sb.append( alphabets[i+2] );
            }
        }
            System.out.println(sb);
        }
}