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.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.StreamTokenizer;


public class C {
	static PrintWriter out;
	static StreamTokenizer st;
	static BufferedReader bf;
	public static void main(String[] args) throws IOException {
		bf = new BufferedReader(new InputStreamReader(System.in));
		st = new StreamTokenizer(bf);
		out = new PrintWriter(new OutputStreamWriter(System.out));
		int n = nextInt();
		int k = nextInt();
		if (k > n || ( n > 1 && k == 1) ){
			System.out.println(-1);
			return;
		}
		char []a = new char [n+1];
		int cnt = 0;
		char x = (char)(96+k);
		while (cnt < k-2){
			a[n] = x;
			n--;
			x--;
			cnt++;
		}
		x = 'a';
		for (int i = 0; i < n; i++) {
			a[i+1] = x++;
			if (x == 'c') x = 'a';
		}
		PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
		for (int i = 1; i < a.length; i++) {
			out.print(a[i]);
		}
		out.close();
	}
	private static String nextLine() throws IOException {
		return bf.readLine().trim();
	}
	private static double nextDouble() throws IOException{
		st.nextToken();
		return st.nval;
	}
	private static long nextLong()  throws IOException{
		st.nextToken();
		return (long) st.nval;
	}
	private static int nextInt() throws IOException {
		st.nextToken();
		return (int) st.nval;
	}

}
