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.*;

public class Main {

	public static void main(String[] args) {
		InputReader in = new InputReader(System.in);
//		int t = in.nextInt();
//		while (t-- > 0) {
			int n = in.nextInt(), k = in.nextInt();
			StringBuilder str = new StringBuilder("");
			if(n < k || (k == 1 && n > 1)) {
				System.out.println(-1);
				return;
			}else if(n == 1 && k == 1) {
				System.out.println('a');
				return;
			}
			for(int i = 0; i < n-k+2; i++) {
				if((i&1) == 0) {
					str.append('a');
				}else {
					str.append('b');
				}
			}
			int c = 'b'+ 1;
			for(int i = 0; i < k-2; i++) {
				str.append((char)(c+i));
			}
			System.out.println(str.toString());
//		}
	}
}


class InputReader {
	public BufferedReader reader;
	public StringTokenizer tokenizer;

	public InputReader(InputStream stream) {
		reader = new BufferedReader(new InputStreamReader(stream), 32768);
		tokenizer = null;
	}

	public String next() {
		while (tokenizer == null || !tokenizer.hasMoreTokens()) {
			try {
				tokenizer = new StringTokenizer(reader.readLine());
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
		}
		return tokenizer.nextToken();
	}

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

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

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

	String nextLine() {
		String str = "";
		try {
			str = reader.readLine();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return str;
	}

}