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.*;
public class code {
	public static void main(String[] args)throws IOException {
		FastReader sc = new FastReader();
		PrintWriter p = new PrintWriter(System.out);


		int n = sc.nextInt();
		int k = sc.nextInt();
		if (k > n || (k == 1 && k != n))  {
			System.out.println(-1);
			return;
		}
		StringBuilder ans = new StringBuilder();
		int j = 0;
		for (char c = 'a'; j < k && c <= 'z'; c++) {
			ans.append(c);
			j++;
		}
		int tmp = n - k;
		StringBuilder q = new StringBuilder();
		if (tmp % 2 == 0) {
			for (int i = 0; i < tmp / 2; i++) {
				q.append("ab");
			}
			System.out.println(q.append(ans));
		} else {
			for (int i = 0; i < tmp / 2; i++) {
				q = q.append("ab");
			}
			StringBuilder r = new StringBuilder(); r.append("aba" + ans.substring(2, ans.length()));
			System.out.println(q.append(r));
		}






	}





}


class FastReader {
	BufferedReader br;
	StringTokenizer st;

	public FastReader() {
		br = new BufferedReader(new
		                        InputStreamReader(System.in));
	}

	String next() {
		while (st == null || !st.hasMoreElements()) {
			try {
				st = new StringTokenizer(br.readLine());
			} catch (IOException  e) {
				e.printStackTrace();
			}
		}
		return st.nextToken();
	}

	int nextInt() {
		return Integer.parseInt(next());
	}

	long nextLong() {
		return Long.parseLong(next());
	}

	double nextDouble() {
		return Double.parseDouble(next());
	}

	String nextLine() {
		String str = "";
		try {
			str = br.readLine();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return str;
	}
}