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.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.StringTokenizer;
	
	
	public class A{
	
		public static void main(String[] args)throws Exception {
			Scanner sc = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			StringBuilder sb = new 	StringBuilder();
			int n = sc.nextInt() , k = sc.nextInt();
			int i ;
			
			if(k == 1 && n > 1 || k > n ) {System.out.println(-1);return;}
			if(n == 1 ) {System.out.println('a');return;}
			for(i = 0 ; i < n-(k-2) ;++i)
				sb.append((i&1) == 0 ? 'a':'b');
			int x = 1;
			for(; i < n ; ++i)
				sb.append((char)('b'+x++));
			out.print(sb.toString());
				
			out.flush();
			out.close();
		}
		
	
		static class Scanner {
			StringTokenizer st;
			BufferedReader br;
	
			public Scanner(InputStream s) {
				br = new BufferedReader(new InputStreamReader(s));
			}
	
			public Scanner(FileReader r) {
				br = new BufferedReader(r);
			}
	
			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();
			}
	
		}
	
	}