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.BufferedWriter;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Scanner;

public class A488 {
	public static String[] thanks = { "vaporeon", "jolteon", "flareon",
			"espeon", "umbreon", "leafeon", "glaceon", "sylveon" };
	public static PrintWriter out;

	public static void main(String[] Args) {
		Scanner sc = new Scanner(System.in);
		out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
				System.out)));

		int n = sc.nextInt();
		int k = sc.nextInt();
		if (n == 1 && k == 1) {
			System.out.println("a");
			return;
		}
		if (k == 1) {
			System.out.println(-1);
			return;
		}
		if (n < k) {
			System.out.println(-1);
			return;
		}
		for (int j = 0; j < n - (k - 2); j++) {
			out.print(j % 2 == 0 ? "a" : "b");
		}
		for (int j = 0; j < k - 2; j++) {
			out.print((char) (j + 'c'));
		}
		out.println();

		out.close();
	}

	private static long choo(int k, int n) {
		if (k < 0 || k > n)
			return 0;
		long ret = fact(n) / (fact(k) * fact(n - k));
		return ret;
	}

	private static long fact(int n) {
		if (n == 0 || n == 1)
			return 1;
		return fact(n - 1) * n;
	}

	public static class Pair implements Comparable<Pair> {
		int t;
		int num;
		String team;
		int t2;

		public int compareTo(Pair o) {
			if (t != o.t)
				return t - o.t;
			return t2 - o.t2;
		}
	}

	public static long comp527A(long a, long b) {
		if (a == 0 || b == 0) {
			return 0;
		}
		if (a > b) {
			return comp527A(b, a);
		}
		return (b / a) + comp527A(b % a, a);
	}
}
