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.util.Scanner;

public class CFA {

	private void work() throws IOException {
		Scanner sc = new Scanner(new BufferedReader(new InputStreamReader(
				System.in)));
		while (sc.hasNextInt()) {
			int n = sc.nextInt();
			int k = sc.nextInt();
			if (n < k || (n > 1 && k == 1)) {
				System.out.println("-1");
			} else {
				if (n == 1) {
					System.out.println("a");
				} else {
					k -= 2;
					n -= k;
					while (n-- > 0) {
						System.out.write('a');
						if (n-- > 0) {
							System.out.write('b');
						}
					}
					for (char c = 'c'; c < (char) ('c' + k); c++) {
						System.out.write(c);
					}
					System.out.println();
				}
			}
		}
		System.out.close();
	}

	public static void main(String[] args) throws IOException {
		new CFA().work();
	}

}
