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.ArrayList;
import java.util.Collections;
import java.util.List;

public class Task288A {

    public static void main(String... args) throws NumberFormatException,
            IOException {
        Solution.main(System.in, System.out);
    }

    static class Scanner {

        private final BufferedReader br;
        private String[] cache;
        private int cacheIndex;

        Scanner(InputStream is) {
            br = new BufferedReader(new InputStreamReader(is));
            cache = new String[0];
            cacheIndex = 0;
        }

        int nextInt() throws IOException {
            if (cacheIndex >= cache.length) {
                cache = br.readLine().split(" ");
                cacheIndex = 0;
            }
            return Integer.parseInt(cache[cacheIndex++]);
        }

        long nextLong() throws IOException {
            if (cacheIndex >= cache.length) {
                cache = br.readLine().split(" ");
                cacheIndex = 0;
            }
            return Long.parseLong(cache[cacheIndex++]);
        }

        String next() throws IOException {
            if (cacheIndex >= cache.length) {
                cache = br.readLine().split(" ");
                cacheIndex = 0;
            }
            return cache[cacheIndex++];
        }

        void close() throws IOException {
            br.close();
        }

    }


    static class Solution {


        public static void main(InputStream is, OutputStream os)
                throws NumberFormatException, IOException {
            PrintWriter pw = new PrintWriter(os);
            Scanner sc = new Scanner(is);

            int n = sc.nextInt();
            int k = sc.nextInt();

            if( n  < k || (k == 1 && n > 1)){
                pw.println(-1);
            } else {
                String ending1 = "abcdefghijklmnopqrstuvwxyz";
                String ending2 = "bacdefghijklmnopqrstuvwxyz";
                for(int i = 0 ; i < n-k ; i++){
                    if(i % 2 == 0){
                        pw.print('a');
                    } else {
                        pw.print('b');
                    }
                }
                if((n-k) % 2 == 0){
                    pw.println(ending1.substring(0,k));
                } else {
                    pw.println(ending2.substring(0,k));
                }
            }


            pw.flush();
            sc.close();
        }
    }

}