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.util.HashSet;
import java.util.Scanner;

public class CString {
	public final char[] ALPHABET = "abcdefghijklmnopqrstuvwxyz".toCharArray();
	public final char[] ALPHABET2 = "ab".toCharArray();

	public static Scanner in = new Scanner(System.in);

	public static void main(String[] args) {
		int length = in.nextInt();
		int unique = in.nextInt();
		String answ = new CString().solve(length, unique);
		System.out.println(answ);
	}

	public String solve(int length, int unique) {
		if(unique > length){return "-1";}
		if(unique > 26){ return "-1";}
		if(unique==1 && length != 1){return "-1";}
		if(unique==1 && length == 1){return "a";}
		StringBuilder b = new StringBuilder();
		b.append('a');
		unique--;length--; 	
		boolean a = false;
		while(unique <= length){
			if(a){
				b.append('a');
			} else {
				b.append('b');
			}
			length--;
			a = !a;
		}
		int i=2;
		while(length>0){
			b.append(ALPHABET[i]);
			i++;
			--length;
		}
		return b.toString();
		
	}

	public String word(StringBuilder f, int n, int k, char l, HashSet<Character> set) {
		if (n == 0 && k == 0) {
			return f.toString();
		}
		
		if (k > n) {
			return "-1";
		}

		String non = findLowestUsed(set, l);
		String s;
		if (non.equals("-1") || k==n) {
			s = "-1";
		} else {
			char nonc = non.charAt(0);
			s = word(f.append(non), n - 1, k, nonc, set);
		}
		if (!s.equals("-1")) {
			return s;
		} else {
			String unu = findLowestUnique(set, l);
			if ("-1".equals(unu)) {
				return "-1";
			}
			
			char unuc = unu.charAt(0);
			set.add(unuc);
			String nexta = word(f.append(unuc), n - 1, k - 1, unuc, set);
			set.remove(unuc);
			return nexta;
		}
	}

	public java.lang.String findLowestUnique(HashSet<Character> s, char lastChar) {
		for (char c : ALPHABET) {
			if (c != lastChar && !s.contains(c))
				return "" + c;
		}
		return "-1";
	}

	public java.lang.String findLowestUsed(HashSet<Character> s, char lastChar) {
		for (char a : ALPHABET) {
			if (s.contains(a) && a!=lastChar) {
				return "" + a;
			}
		}
		return "-1";
	}
}
