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.InputStream;
import java.io.PrintWriter;
import java.io.DataInputStream;
import java.util.Arrays;


public class Polo_the_penguin_Strings{

	public static void main(String[] args) throws Exception {
		ParserdoubtR2 sc = new ParserdoubtR2(System.in);
		PrintWriter pw = new PrintWriter(System.out);

		int n = sc.nextInt();
		int k = sc.nextInt();
		int length = n-k+2; 
		StringBuffer str= new StringBuffer("ab");
		String st = "cdefghijklmnopqrstuvwxyz";
		if(k>n)
		{
			pw.println(-1);
		}else if(k==1)
		{
			if (n==1)
				pw.println("a");
			else
			pw.println(-1);
		}else
			
		{
		
			while(str.length()<length)
			{
				str.append(str.toString());
			}
			
			pw.println((str.substring(0, length)  + st.substring(0,k-2)));
			
		}	

		

		pw.close();

	}

}










class ParserdoubtR2 {
	final private int BUFFER_SIZE = 1 << 19;

	private DataInputStream din;
	private byte[] buffer;
	private int bufferPointer, bytesRead;

	public ParserdoubtR2(InputStream in) {
		din = new DataInputStream(in);
		buffer = new byte[BUFFER_SIZE];
		bufferPointer = bytesRead = 0;
	}

	public String nextString() throws Exception {
		StringBuffer sb = new StringBuffer("");
		byte c = read();
		while (c <= ' ')
			c = read();
		do {
			sb.append((char) c);
			c = read();
		} while (c > ' ');
		return sb.toString();
	}

	public char nextChar() throws Exception {
		byte c = read();
		while (c <= ' ')
			c = read();
		return (char) c;
	}

	public int nextInt() throws Exception {
		int ret = 0;
		byte c = read();
		while (c <= ' ')
			c = read();
		boolean neg = c == '-';
		if (neg)
			c = read();
		do {
			ret = ret * 10 + c - '0';
			c = read();
		} while (c > ' ');
		if (neg)
			return -ret;
		return ret;
	}

	public long nextLong() throws Exception {
		long ret = 0;
		byte c = read();
		while (c <= ' ')
			c = read();
		boolean neg = c == '-';
		if (neg)
			c = read();
		do {
			ret = ret * 10 + c - '0';
			c = read();
		} while (c > ' ');
		if (neg)
			return -ret;
		return ret;
	}

	private void fillBuffer() throws Exception {
		bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
		if (bytesRead == -1)
			buffer[0] = -1;
	}

	private byte read() throws Exception {
		if (bufferPointer == bytesRead)
			fillBuffer();
		return buffer[bufferPointer++];
	}
}