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.*;
import java.util.StringTokenizer;

/**
 * Created by Alvin on 8/8/2016.
 */

public class Codeforces_round_177_div_1_PoloThePenguinAndStrings {
    public static void main(String[] args) {
        FScanner input = new FScanner();
        out = new PrintWriter(new BufferedOutputStream(System.out), true);
        int length = input.nextInt();
        int charCount = input.nextInt();
        if (charCount > length || (charCount == 1 && length > 1)) {
            out.println(-1);
        } else {
            StringBuilder solution = new StringBuilder();
            solution.append('a');
            for (int i = 2; i <= length; i++) {
                solution.append(i % 2 == 0 ? 'b' : 'a');
            }
            char next = 'c';
            if (charCount > 2) {
                for (int i = length - charCount + 2; i < length; i++) {
                    solution.setCharAt(i, next);
                    next = (char)((int)next + 1);
                }
            }
            out.println(solution);
        }
        out.close();
    }

    public static PrintWriter out;

    public static class FScanner {
        BufferedReader br;
        StringTokenizer st;

        public FScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        private String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

    }
}

