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.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Scanner;


public class task_177_C {
	static Scanner sc;
	static int a[];
	static boolean used[];
public static void main(String[] args) {
  sc = new Scanner(System.in);
  int n =sc.nextInt();
  int k =sc.nextInt();
  PrintWriter pw =new PrintWriter(new OutputStreamWriter(System.out));
  if(n==1 && k==1){
	  pw.print("a");
	  pw.close();
	  return;
  }
  if(n<k || k==1)pw.println(-1);
  else
  {   int x=0;
	  
	   x=(n-(k-2))/2;
	  for (int i = 1; i <=x; i++) {
		pw.print("ab");
	  }
	  if((n-(k-2))%2!=0)
		  pw.print("a");
	  k=k-2;
	  for (char j ='c'; j <='z'; j++) {
		  if(k<=0)
			  break;
		pw.print(j);
		k--;
	
	  }
  }
  pw.close();
}
//private static void outarray(int n) {for (int i=1;i<=n;i++)System.out.print(a[i]+" ");}
//private static void inarray(int n) { for (int i = 1; i <=n; i++)a[i]=sc.nextInt();}
//private static void prime(int x){for (int i = 2; i <=x; i++) {if(!used[i]){int k=2;	while(k*i<=1000000){used[k*i]=true;	k++;}}}}
}
