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 Main {
	
	Scanner in;
	static PrintWriter out;
	
	
	
	void solve() {
		int len = in.nextInt();
		int count = in.nextInt();
		if (count == 1) {
			if (len != 1) {
				out.println(-1);
				return;
			}
		}
		if (count > len) {
			out.println(-1);
			return;
		}
		char []str = new char[len];
		for (int i = 0; i < str.length; i++) {
			if (i % 2 == 0) {
				str[i] = 'a';
			} else {
				str[i] = 'b';
			}
		}
		
		for (int i = 3; i <= count; i++) {
			str[len - count + i - 1] = (char)('a' + i - 1);
		}
		
		for (int i = 0; i < len; i++) {
			out.print(str[i]);
		}
	}
	
	
	static void check(boolean e) {
		if (!e) {
			throw new Error();
		}
	}
	
	
	public void run() {
		in = new Scanner(System.in);
		out = new PrintWriter(System.out);
		
		try {
			solve();
		} finally {
			out.close();
		}
	}
	
	public static void main(String[] args) {
		new Main().run();
	}

} 