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.*;

import javax.swing.plaf.basic.BasicScrollPaneUI.HSBChangeListener;

public class Main {

    static BufferedReader br;
    static PrintWriter out;
    static StringTokenizer st;
    
    static void solve() throws Exception {
        int n = nextInt();
        int k = nextInt();
        if(k > n){
            out.println("-1");
            return;
        }
        if(k == 1 && n != 1){
            out.println("-1");
            return;
        }
        char[] s = new char[n];
        for(int i = 0; i < Math.min(n, n - k + 2); i++){
            if(i % 2 == 0){
                s[i] = 'a';
            }else{
                s[i] = 'b';
            }
        }
        for(int i = n - k + 2, p = 1; i < n; i++, p++){
            s[i] = (char) ('b' + p);
        }
        String ans = new String(s);
        out.print(ans);
    }

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

    static long nextLong() throws IOException {
        return Long.parseLong(next());
    }

    static double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

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

    public static void main(String[] args) {
        try {
            InputStream input = System.in;
            OutputStream output = System.out;
            br = new BufferedReader(new InputStreamReader(input));
            out = new PrintWriter(new PrintStream(output));
            solve();
            out.close();
            br.close();
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }
}