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.lang.reflect.Array;
import java.util.*;

public class Main {
    public static void main(String[] args) throws IOException {
        InputReader in = new InputReader();
        PrintWriter out = new PrintWriter(System.out);
        int test_cases = 1;
        Solver s = new Solver();
        for (int i = 1; i <= test_cases; i++) {
            s.solve(i, in, out);
        }
        out.close();
    }
}

class Solver {
    void solve(int test_number, InputReader in, PrintWriter out) throws IOException {
        int n = in.nextInt();
        int k = in.nextInt();
        if (k > n || (k == 1 && n > 1)) {
            out.println(-1);
            return;
        }
        if (n == 1) {
            if (k > 1) {
                out.println(-1);
            } else {
                out.print("a");
            }
            return;
        }
        for (int i = 0; i < n - k + 2; i++) {
            if (i % 2 == 0) out.print("a");
            else out.print("b");
        }
        int ch = 99;
        for (int i = n - k + 2; i < n; i++) {
            out.print((char) (ch++));
        }
    }
}


class InputReader {
    BufferedReader br;
    StringTokenizer st;

    public InputReader()
    {
        br = new BufferedReader(new
                InputStreamReader(System.in));
    }

    String next()
    {
        while (st == null || !st.hasMoreElements())
        {
            try
            {
                st = new StringTokenizer(br.readLine());
            }
            catch (IOException  e)
            {
                e.printStackTrace();
            }
        }
        return st.nextToken();
    }

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

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

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

    String nextLine()
    {
        String str = "";
        try
        {
            str = br.readLine();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
        return str;
    }
}