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.StringTokenizer;

public class strings {
    static int n, k;
    static char alpha[];
    
    public static void main(String[] args) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        //BufferedReader in = new BufferedReader(new FileReader("strings.in"));
        
        StringTokenizer nk = new StringTokenizer(in.readLine());
        n = Integer.parseInt(nk.nextToken());
        k = Integer.parseInt(nk.nextToken());
        
        alpha = new char[26];
        for (int i = 0; i < 26; i++) alpha[i] = (char) (i+97);
        
        if (k > n) {
            System.out.println(-1);
            return;
        }
        
        if (k == 1) {
            if (n == 1) System.out.print("a");
            else System.out.print(-1);
            return;
        }
        
        for (int i = 0; i < n-k+2; i++) System.out.print(alpha[i%2]);
        for (int i = 0; i < k-2; i++) System.out.print(alpha[(i+2)%26]);
    }
}