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 Main{

    public void run(){
	Scanner in = new Scanner(System.in);
	PrintWriter out = new PrintWriter(System.out);
	
	int n = in.nextInt();
	int k = in.nextInt();
	
	if (k > n){
	    System.out.println(-1);
	    return;
	}

	if (k == 1 && n > 1){
	    System.out.println(-1);
	    return;
	}

	if (k == 1 && n == 1){
	    System.out.println('a');
	    return;
	}

	for(int i=0;i<n-k+2;i++){
	    if (i % 2 == 0) out.print('a');
	    else out.print('b');
	}

	for(int i=0;i<k-2;i++){
	    out.print((char)('c'+i));
	}
	out.println();
	out.close();
    }
    
    public static void main(String args[]){
	new Main().run();
    }
}