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.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.StringTokenizer;

import java.util.Scanner;


public class C {
	
	public static void main(String[] args) throws IOException{
		Scanner sc = new Scanner(System.in);
		
		final int n = sc.nextInt();
		final int k = sc.nextInt();
		
		if(n < k){
			System.out.println(-1);
			return;
		}
		
		if(k == 1){
			if(n == 1){
				System.out.println("a");
				return;
			}else{
				System.out.println(-1);
				return;
			}
		}
		
		StringBuilder sb = new StringBuilder();
		final int rest = k - 2;
		for(int i = 0; i < n - rest; i++){
			sb.append(i % 2 == 0 ? 'a' : 'b');
		}
		
		for(int i = 0; i < rest; i++){
			sb.append((char)('c' + i));
		}
		
		System.out.println(sb.toString());
	}
	
	public static class Scanner {
		
		private BufferedReader br;
		private StringTokenizer tok;
		
		public Scanner(InputStream is) throws IOException{
			br = new BufferedReader(new InputStreamReader(is));
			getLine();
		}
		
		private void getLine() throws IOException{
			tok = new StringTokenizer(br.readLine());
		}
		
		private boolean hasNext(){
			return tok.hasMoreTokens();
		}
		
		public int nextInt() throws IOException{
			if(hasNext()){
				return Integer.parseInt(tok.nextToken());
			}else{
				getLine();
				return Integer.parseInt(tok.nextToken());
			}
		}
		
		public long nextLong() throws IOException{
			if(hasNext()){
				return Long.parseLong(tok.nextToken());
			}else{
				getLine();
				return Long.parseLong(tok.nextToken());
			}
		}
		
		public double nextDouble() throws IOException{
			if(hasNext()){
				return Double.parseDouble(tok.nextToken());
			}else{
				getLine();
				return Double.parseDouble(tok.nextToken());
			}
		}
	}
}