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.lang.*;
import java.io.*;

public class polo
   {
  public static void main(String[] args) throws IOException
   {     
    BufferedReader cin=new BufferedReader(new InputStreamReader(System.in));
    String s=cin.readLine();
    String[] tok=s.split(" ");
    int n=Integer.parseInt(tok[0]);
    int t=Integer.parseInt(tok[1]);
    if(n<t)
    System.out.println("-1");
    else if(t==1)
     {
        if(n==1)
          System.out.println("a");
        else
          System.out.println("-1");
     }        
    else
    {
    for(int i=1;i<=n-(t-2);i++)
     {
        if(i%2==0)
            System.out.print("b");
        else    
            System.out.print("a");
     }  
    char c='c';  
     for(int i=0;i<t-2;i++)
      {
         System.out.print(c);
         c++;
      }
}
} 
}     