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.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.ListIterator;
import java.util.Locale;
import java.util.Random;
import java.util.Stack;
import java.util.StringTokenizer;

public class B {

	
	private void solve() throws IOException {
		int n = nextInt();
		int k = nextInt();
		
		if (k == 1) {
			if (n == 1) {
				println("a");
			} else {
				println("-1");
			}
			return;
		}
		
		
		if (k > n) {
			println("-1");
			return;
		}
		
		while (n > k - 1) {
			print("ab");
			n -= 2;
		}
		
		if (n == k - 1) {
			print("a");
		}
		
		for (int i = 0; i < k - 2; i++) {
			print((char)(i + 'a' + 2));
		}
		
		println("");
		
	}
	

	private String nextToken() throws IOException {
		while (tokenizer == null || !tokenizer.hasMoreTokens()) {
			tokenizer = new StringTokenizer(reader.readLine());
		}
		return tokenizer.nextToken();
	}

	private int nextInt() throws NumberFormatException, IOException {
		return Integer.parseInt(nextToken());
	}

	private double nextDouble() throws NumberFormatException, IOException {
		return Double.parseDouble(nextToken());
	}

	private long nextLong() throws IOException {
		return Long.parseLong(nextToken());
	}

	private void print(Object o) {
		writer.print(o);
	}

	private void println(Object o) {
		writer.println(o);
	}

	private void printf(String format, Object... o) {
		writer.printf(format, o);
	}

	public static void main(String[] args) {
		long time = System.currentTimeMillis();
		Locale.setDefault(Locale.US);
		new B().run();
		System.err.printf("%.3f\n", 1e-3 * (System.currentTimeMillis() - time));
	}

	BufferedReader reader;
	StringTokenizer tokenizer;
	PrintWriter writer;

	private void run() {
		try {
			reader = new BufferedReader(new InputStreamReader(System.in));
			writer = new PrintWriter(System.out);

			solve();
			reader.close();
			writer.close();
		} catch (IOException e) {
			e.printStackTrace();
			System.exit(13);
		}
	}
}