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.*;
import java.io.*;
public class j {
public static void main(String[] args) throws Exception
{
    Scanner input = new Scanner(System.in);
    int n = input.nextInt(), k = input.nextInt();
    PrintWriter out = new PrintWriter(System.out);
    if(k>n) out.println(-1);
    else if(k==1)
    {
        if(n==1) out.print('a');
        else out.println(-1);
    }
    else
    {
        int x = n - k+2;
        for(int i = 0; i<x/2; i++) out.print("ab");
        if(x%2==1) out.print("a");
        for(int i = 0; i<k-2; i++) out.print((char)('c'+i));
    }
    out.close();
}
}
