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.PrintWriter;
import java.util.Scanner;

public class B177 {

    public void solve() throws IOException {

        Scanner sc = new Scanner(System.in);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw = new PrintWriter(System.out);


        int i = 0, j = 0;
        int ans = 0, sum = 0, count = 0;
        String s[] = br.readLine().split(" ");
        int n = Integer.parseInt(s[0]);
        int k = Integer.parseInt(s[1]);

        StringBuilder sb = new StringBuilder();
        StringBuffer sbuf = new StringBuffer();
        String temp = "ab";
        String temp2 = "";

        if (k == 1) {
            if (n > k) {
                System.out.println("-1");
                System.exit(0);
            } else {
                for (int l = 0; l < n; l++) {
                    //  sb.append("a");
                    sbuf.append("a");
                }
            }
//            System.out.println(sb.toString());
            pw.write(sbuf.toString());
        } else if (n > k) {
            for (int l = 0; l < k; l++) {
                temp2 += (char) ('a' + l);
            }
            int num = 0;
            // if (n % 2 != 0) {
            //num = n - (n - (k - 1));
            num = (n - (k - 1)) / 2;
            // System.out.println(num);
            for (int l = 0; l < num; l++) {
                //  sb.append(temp);
                sbuf.append(temp);
            }
             
            if (sbuf.length() + temp2.length() != n) {
                // sb.append(temp2.replaceAll("b", ""));
              //  System.out.println("jj");
                sbuf.append(temp2.replaceAll("b", ""));
            } else {
                // sb.append(temp2);
                sbuf.append(temp2);
            }
//            System.out.println(sb.toString());
            pw.write(sbuf.toString());


        } else if (n == k) {
            for (int l = 0; l < k; l++) {
                temp2 += (char) ('a' + l);
            }
            pw.write(temp2);

        } else {
            pw.write("-1");
        }
        pw.flush();
        pw.close();
    }

    public static void main(String[] args) throws IOException {
        new B177().solve();
        //System.out.println(126%26);
    }
}
