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 A implements Runnable {

	private void solve() throws IOException {
		int n = nextInt(), k = nextInt();
		if (k == 1 && n == 1) {
			print('a');
			return;
		}
		if (k == 1 || k > n) {
			print(-1);
			return;
		}
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n - k + 2; i++)
			if ((i & 1) == 0)
				sb.append('a'); else sb.append('b');
		for (int i = 2; i < k; i++)
			sb.append((char) (i + 'a'));
		print(sb.toString());
	}

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

	BufferedReader reader;
	StringTokenizer tokenizer;
	PrintWriter writer;

	public void run() {
		try {
			reader = new BufferedReader(new InputStreamReader(System.in));
			writer = new PrintWriter(System.out);
			solve();
			writer.close();
		} catch (Exception e) {
			e.printStackTrace();
			System.exit(261);
		}
	}

	void halt() {
		writer.close();
		System.exit(0);
	}

	void print(Object... objects) {
		for (int i = 0; i < objects.length; i++) {
			if (i != 0)
				writer.print(' ');
			writer.print(objects[i]);
		}
	}

	void println(Object... objects) {
		print(objects);
		writer.println();
	}

	String nextLine() throws IOException {
		return reader.readLine();
	}

	String nextToken() throws IOException {
		while (tokenizer == null || !tokenizer.hasMoreTokens())
			tokenizer = new StringTokenizer(nextLine());
		return tokenizer.nextToken();
	}

	int nextInt() throws NumberFormatException, IOException {
		return Integer.parseInt(nextToken());
	}
}
