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.lang.StringBuilder;

public class PoloStringsWithStringBuilder
{
    public static void main(String[] args) 
    {
        // Set up scanner
        Scanner sc = new Scanner(System.in); 
        // System.out.println("Input n");
        int n = sc.nextInt();
        // System.out.println("Input k");
        int k = sc.nextInt();
        
        if (k==1 && n==1)
        {
            System.out.println("a");
            return;
        }
            
        if (k>n || k == 1)
        {
            System.out.println(-1);
            return;
        }
                
        StringBuilder answer = new StringBuilder(100000);
        
        int cyclenum = n - k + 2;
        
        for (int i=0; i<cyclenum/4; i++)
        {
            answer.append("abab");
        }
        
        if (cyclenum % 4 == 3)
        {
            answer.append("aba");
        }
        else if (cyclenum % 4 == 2)
        {
            answer.append("ab");
        }
        else if (cyclenum % 4 == 1)
        {
            answer.append("a");
        }
        
        
        int rest = k-2;
        if (rest <= 0)
        {
            System.out.println(answer);
            return;
        }
        char ch = 'c';
        for (int j=0; j<rest; j++)
        {
            answer.append(ch);
            ch++;
        }
        System.out.println(answer.toString());
    }
}
            
        