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.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class B {

    static BufferedReader in;
    static StringTokenizer st;
    static PrintWriter out;

    static String next() throws IOException {
        while (st == null || !st.hasMoreTokens()) {
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }

    static int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    public static void main(String[] args) throws IOException {
        in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
                System.out)));
        int n = nextInt();
        int k = nextInt();
        if (n == 1 && k == 1) {
            out.print('a');
            out.close();
            return;
        }
        if (n < k || k == 1) {
            out.print(-1);
        } else {
            if (n == k) {
                int cnt = 0;
                for (char i = 'a'; i <= 'z'; i++) {
                    out.print(i);
                    cnt++;
                    if (cnt == n) {
                        break;
                    }
                }
            } else {
                int cnt = 0;
                while (cnt != n - k + 2) {
                    if (cnt % 2 == 0) {
                        out.print('a');
                    } else {
                        out.print('b');
                    }
                    cnt++;
                }
                if (cnt < n) {
                    for (char i = 'c'; i <= 'z'; i++) {
                        out.print(i);
                        cnt++;
                        if (cnt == n) {
                            break;
                        }
                    }
                }
            }
        }
        out.close();
    }
}