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.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.io.*;
import java.math.*;
import java.text.*;
import java.util.*;

/*
		br = new BufferedReader(new FileReader("input.txt"));
		pw = new PrintWriter(new BufferedWriter(new FileWriter("output.txt")));
		br = new BufferedReader(new InputStreamReader(System.in));
		pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

 */

public class Main {
	private static BufferedReader br;
	private static StringTokenizer st;
	private static PrintWriter pw;

	static int[] dx = new int[]{-1,0,1,0};
	static int[] dy = new int[]{0,1,0,-1};

	public static void main(String[] args) throws IOException	{
		br = new BufferedReader(new InputStreamReader(System.in));
		pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
		//int qq = 1;
		int qq = Integer.MAX_VALUE;
		//int qq = readInt();
		for(int casenum = 1; casenum <= qq; casenum++)	{
			int n = readInt();
			int k = readInt();
			if(k > n)	{
				pw.println(-1);
			}
			else	{
				char[] list = new char[n];
				for(int i = 0; i < n; i++)	{
					list[i] = i%2==0?'a':'b';
				}
				int index = n-1;
				for(int a = k-1; a >= 2; a--)	{
					list[index--] = (char)('a'+a);
				}
				boolean valid = true;
				for(int i = 0; i < n-1; i++)	{
					if(list[i] == list[i+1]){
						valid = false;
					}
				}
				Set<Character> set = new HashSet<Character>();
				for(char out: list)	{
					set.add(out);
				}
				if(set.size() != k)	{
					valid = false;
				}
				if(valid)	{
					pw.println(new String(list));
				}
				else	{
					pw.println(-1);
				}
			}
		}
		pw.close();
	}

	private static void exitImmediately()	{
		pw.close();
		System.exit(0);
	}

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

	private static double readDouble() throws IOException	{
		return Double.parseDouble(nextToken());
	}

	private static int readInt() throws IOException	{
		return Integer.parseInt(nextToken());
	}

	private static String nextToken() throws IOException	{
		while(st == null || !st.hasMoreTokens())	{
			if(!br.ready())	{
				exitImmediately();
			}
			st = new StringTokenizer(br.readLine().trim());
		}
		return st.nextToken();
	}
}
