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.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class C {

	static BufferedReader br = new BufferedReader(new InputStreamReader(
			System.in));

	static StringTokenizer sc;

	static String nxtTok() throws IOException {
		while (!sc.hasMoreTokens())
			sc = new StringTokenizer(br.readLine());
		return sc.nextToken();
	}

	static int nxtInt() throws IOException {
		return Integer.parseInt(nxtTok());
	}

	public static void main(String[] args) throws IOException {
		PrintWriter out = new PrintWriter(System.out);
		sc = new StringTokenizer(br.readLine());
		int n = nxtInt();
		int k = nxtInt();
		if (k > n || (k == 1 && n > 1))
			out.print(-1);
		else if (n == k) {
			for (int i = 0; i < k; i++)
				out.print((char) (i + 'a'));
		} else {
			for (int i = 0; i < n - k + 2; i++) {
				if (i % 2 == 0)
					out.print("a");
				else
					out.print("b");
			}
			char ch = 'c';
			for (int i = n - k + 2; i < n; i++) {
				out.print(ch);
				ch++;
			}
		}
		out.println();
		br.close();
		out.close();
	}

}
