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.*;
import java.math.*;
public class Main{
public static void main(String[] args) throws IOException {
    BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
    StringBuilder output = new StringBuilder();
    String line, lines[];
    int n, k;
    StringBuilder cad = new StringBuilder(), abc = new StringBuilder();
    for ( int i = 0; i < 500002; i++ )
    {
        cad.append("ab");
    }
    for ( int i = 2; i < 27; i++ )
    {
        abc.append((char)(97+i));
    }
    lines = input.readLine().split(" ");
    n = Integer.parseInt(lines[0]);
    k = Integer.parseInt(lines[1]);
    if ( (k == 1 && n != 1) || k > n)
    {
        System.out.println("-1");
    }
    else if ( k != 1)
    {
        System.out.println(cad.substring(0, n- k + 2)+abc.substring(0, k - 2 ));
    }
    else
    {
        System.out.println("a");
    }
    
}
}
