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.*;

public class Main{
	
	
	static String getAnswer(int n, int k){
		if(n<k) return "-1";
		if(k==1 && n>1) return "-1";
		if(k==1 && n==1) return "a";
		
		int lengthBack = k-2;
		StringBuffer sbBack = new StringBuffer();
		for(int i=0; i<lengthBack; ++i){
			int ascii = (int)'c' + i;
			char c = (char)ascii;
			sbBack.append(c);		
		}
		
		int lengthFront = n-(k-2);
		StringBuffer sbFront = new StringBuffer();
		for(int i=0; i<lengthFront; ++i){
			if(i%2==0){
				sbFront.append('a');
			}
			else{
				sbFront.append('b');
			}
		}
		
		sbFront.append(sbBack);
		
		String res = sbFront.toString();
		return res;
	}
	
	public static void main(String[] args) {
		MyScanner sc = new MyScanner();
		int n = sc.nextInt();
		int k = sc.nextInt();
		
		System.out.print(getAnswer(n,k));
	}
	
	
	
	
	//-----------MyScanner class for faster input----------------------------------
	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}
		
		String nextLine(){
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

		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());
		}
                

	}
	//----------------------------------------------------------------------------
}