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 C {
	public static void main(String[] args) throws NumberFormatException, IOException, Exception {
		BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter out = new PrintWriter(System.out);
		Scanner s = new Scanner(System.in);
		
		String[] split = f.readLine().split("\\s+");
		int n = Integer.parseInt(split[0]), k = Integer.parseInt(split[1]);
		if(k > n || (k == 1 && n > 1)) {
			System.out.println(-1);
			return;
		}
		if(k == 1) {
			System.out.println('a');
			return;
		}
		StringBuilder sb = new StringBuilder();
		sb.append("ab");
		int used = 2;
		while(used < n-(k-2)) {
			if(used%2 == 0) sb.append('a');
			else sb.append('b');
			used++;
		}
		char app = 'c';
		while(used < n) {
			sb.append(app);
			app++;
			used++;
		}
		System.out.println(sb.toString());
	}
}