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.*;
import java.math.*;
import java.awt.geom.*;

import static java.lang.Math.*;

public class Solution implements Runnable {
	
	public void solve () throws Throwable {
		int n = sc.nextInt();
		int k = sc.nextInt();
		if (k == 1) {
			if (n == 1) {
				out.println("a");
			} else {
				out.println("-1");
			}
			return;
		}
		if (k > n) {
			out.println("-1");
			return;
		}
		for (int i = 0; i < n - (k - 2); i++) {
			if (i % 2 == 0) {
				out.print("a");
			} else {
				out.print("b");
			}
		}
		for (int i = 0; i < k - 2; i++) {
			char c = (char)(i + 'c');
			out.print(c);
		}
		out.println();
	}

	BufferedReader in;
	PrintWriter out;
	FastScanner sc;

	static Throwable uncaught;
	
	@Override
	public void run() {
		try {
			in = new BufferedReader(new InputStreamReader(System.in));
			out = new PrintWriter(System.out);
			sc = new FastScanner(in);
			solve();
		} catch (Throwable t) {
			Solution.uncaught = t;
		} finally {
			out.close();
		}
	}

	public static void main(String[] args) throws Throwable {
		Thread t = new Thread(null, new Solution(), "", (1 << 26));
		t.start();
		t.join();
		if (uncaught != null) {
			throw uncaught;
		}
	}

}

class FastScanner {

	BufferedReader reader;
	StringTokenizer strTok;

	public FastScanner(BufferedReader reader) {
		this.reader = reader;
	}

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

	public int nextInt() throws IOException {
		return Integer.parseInt(nextToken());
	}

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

	public double nextDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}

}