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.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class C {

	private static StringTokenizer tokenizer;
    private static BufferedReader bf;
    private static PrintWriter out;
    
	private static int nextInt() throws IOException {
    	return Integer.parseInt(nextToken());
    }
    
    @SuppressWarnings("unused")
	private static long nextLong() throws IOException {
    	return Long.parseLong(nextToken());
    }
    
    private static String nextToken() throws IOException {
    	while(tokenizer == null || !tokenizer.hasMoreTokens()) {
    		tokenizer = new StringTokenizer(bf.readLine());
    	}
    	return tokenizer.nextToken();
    }
    
	public static void main(String[] args) throws IOException {
		
		bf = new BufferedReader(new InputStreamReader(System.in));
    	tokenizer = null;
    	out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    	int n = nextInt(); int k = nextInt();
    	if(k > n || k > 26 || (k == 1 && n > 1)) out.println(-1);
    	else {
    		int m = n; int mod = 0;
    		while(m > Math.max(0, k-2)) {
    			if(mod %2 == 0) out.print("a");
    			else out.print("b");
    			m--; mod += 1;
    		}
    		char s = 'c';
    		for(int i = m; i >= 1; i--) {
    			out.print(Character.toChars(s + (m-i)));
    		}
    	}
    	out.close();
	}

}
