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.Arrays;
import java.util.Random;
import java.util.StringTokenizer;

public class Main {
    FastScanner in;
    PrintWriter out;

    public void solve() throws IOException {

        int n = in.nextInt();
        int k = in.nextInt();
        char[] result = new char[n];
//        Random rng = new Random();
        if (k > n || (k == 1 && n != 1)){
            out.print(-1);
            return;
        }
        if (k == 1 && n == 1){
            out.print("a");
            return;
        }
        boolean a = true;
        for (int i = 0; i < n - k + 2; i++){
            if (a)
                result[i] = 'a';
            else
                result[i] = 'b';
            a = !a;
        }
        for (int i = 0; i < k - 2; i++){
            result[i + n - k + 2] = (char)((int)('c') + i);
        }
//        for (int i=0; i<n; i++){
//            int j = rng.nextInt(i + 1);
//            long tmp = array[j];
//            array[j] = array[i];
//            array[i] = tmp;
//        }
//        Arrays.sort(array);

        out.print(result);

//        out.flush();
    }

    private boolean check(int turns, int x, int y, int x0, int y0) {
        if (x0==x && y0==y){
            out.println(turns);
            return true;
        }
        return false;
    }

    public void run() {
        try {
//            in = new FastScanner(new File("Proba2.in"));
            in = new FastScanner();
//            out = new PrintWriter(new File(".out"));
            out = new PrintWriter(System.out);

            solve();

            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;

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

        FastScanner(File f) {
            try {
                br = new BufferedReader(new FileReader(f));
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }

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

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

    public static void main(String[] arg) {
        new Main().run();
    }
}