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 static java.lang.Math.*; 

import java.io.*; 
import java.util.*; 
import java.math.*;

public class Template {
	BufferedReader in; 
	PrintWriter out;
	StringTokenizer st; 

	String next() {
		while (st==null || !st.hasMoreTokens()) {
			try {
				st = new StringTokenizer(in.readLine());
			} catch (Exception e) {}
		}
		return st.nextToken(); 
	}

	int nextInt() {
		return Integer.parseInt(next()); 
	}

	long nextLong() {
		return Long.parseLong(next());
	}

	double nextDouble() {
		return Double.parseDouble(next());
	}

	public void run() throws Exception {
		in = new BufferedReader(new InputStreamReader(System.in));
		out = new PrintWriter(System.out);
		int n = nextInt(), k = nextInt();
		if(n == 1 && k == 1){
			System.out.println('a');
			System.exit(0);
		}
		if(k > n || (k == 1 && n > 1)){
			System.out.println(-1);
			System.exit(0);
		}		
		else{
			StringBuilder sb = new StringBuilder();
			String end = "";
			for(int i = 2; i < k; ++i){
				end += Character.toString((char)(i + 97));
			}
			int m = n - k + 2;
			for(int i = 0; i < m; ++i)
				if(i % 2 == 0) sb.append('a');
				else sb.append('b');
			sb.append(end);
			System.out.println(sb.toString());	
		}
		out.close(); 
	}
	

	public static void main(String[] args) throws Exception {
		new Template().run(); 
	}
}