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.Scanner;

public class PoloThePenguinAndStrings {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int x = input.nextInt();
        int c = input.nextInt();
        if (x!=1 &&c==1) {
            System.out.println(-1);
            return;
        }
        String s = "";
        if (c > x) {
            System.out.println(-1);
            return;
        }
        if (c == 1) {
            for (int i = 0; i < x; i++) {
//              s += "a";
                System.out.print("a");
            }
//          System.out.println(s);
            return;
        }
        int v = c - 2;

        int t = -1;
        int y = x - v;
        for (int i = 0; i < y; i++) {
            t *= -1;
            if (t == 1) {
//              s += "a";
                System.out.print("a");
            } else {
//              s += "b";
                System.out.print("b");
            }
        }

        for (int i = 0; i < v; i++) {
            char b = (char) ('c' + i);
            System.out.print(b);
//          s += b;
        }
//      System.out.println(s);
    }

}
