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.io.PrintWriter;
import java.util.Arrays;
import java.util.Comparator;
	import java.util.PriorityQueue;
	import java.util.StringTokenizer;
	
	public class Abood2A {
		
		static long GCD(long x, long y) {
			   if (y==0) return x;
			   return GCD(y,x%y);
			}
	
		public static void main(String[] args) throws IOException {
			Scanner sc = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
	
			int n = sc.nextInt();
			int k = sc.nextInt();
			
			if(n == 1 && k == 1){
				System.out.println("a");
				return;
			}
			
			if(n == 1 || k == 1){
				System.out.println(-1);
				return;
			}
			
			char[] ans = new char[n];
			
			for (int i = 0; i < n; i++) {
				if(i%2 == 0)
					ans[i] = 'a';
				else
					ans[i] = 'b';
			}
			
			int r = k - 1;
			int p = n - 1;
			while(r > 1) {
				if(p < 2){
					System.out.println(-1);
					return;	
				}
				ans[p--] = (char) ('a' + r);
				r--;
			}
			
			for (int i = 0; i < ans.length; i++) {
				out.print(ans[i]);
			}
			out.println();
			out.flush();
			out.close();
		}
	
		static class Scanner 
		{
			StringTokenizer st;
			BufferedReader br;
	
			public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}
	
			public String next() throws IOException 
			{
				while (st == null || !st.hasMoreTokens()) 
					st = new StringTokenizer(br.readLine());
				return st.nextToken();
			}
	
			public int nextInt() throws IOException {return Integer.parseInt(next());}
	
			public long nextLong() throws IOException {return Long.parseLong(next());}
	
			public String nextLine() throws IOException {return br.readLine();}
	
			public double nextDouble() throws IOException
			{
				String x = next();
				StringBuilder sb = new StringBuilder("0");
				double res = 0, f = 1;
				boolean dec = false, neg = false;
				int start = 0;
				if(x.charAt(0) == '-')
				{
					neg = true;
					start++;
				}
				for(int i = start; i < x.length(); i++)
					if(x.charAt(i) == '.')
					{
						res = Long.parseLong(sb.toString());
						sb = new StringBuilder("0");
						dec = true;
					}
					else
					{
						sb.append(x.charAt(i));
						if(dec)
							f *= 10;
					}
				res += Long.parseLong(sb.toString()) / f;
				return res * (neg?-1:1);
			}
	
			public boolean ready() throws IOException {return br.ready();}
	
	
		}
	
	}