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.PrintWriter;
import java.util.Arrays;
import java.util.Scanner;

public class Problem3 implements Runnable {

    public void run() {
        Scanner scanner = new Scanner(System.in);
        PrintWriter writer = new PrintWriter(System.out);

        int n  = scanner.nextInt();
        int k = scanner.nextInt();
        if (n == 1 && k == 1) {
            writer.println("a");
            writer.close();
            return;
        }
        if ((n > 1) && (k == 1)) {
            writer.println(-1);
            writer.close();
            return;
        }
        if (n < k) {
            writer.println(-1);
        } else {
            char ch = 'a';
            for (int i = 0; i < n - k + 2; i++) {
                writer.print(ch);
                if (ch == 'a') {
                    ch = 'b';
                } else {
                    ch = 'a';
                }
            }
            String endLine = "";
            ch = 'c';
            for (int i = 0; i < k - 2; i++) {
                endLine += ch;
                ch++;
            }

            writer.println(endLine);
        }

        writer.close();
    }

    public static void main(String[] args) {
        new Problem3().run();
    }
}
