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.IOException;
import java.io.OutputStreamWriter;
import java.math.BigDecimal;
import java.io.BufferedWriter;
import java.util.Locale;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.math.BigInteger;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author Jacob Jiang
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		InputReader in = new InputReader(inputStream);
		OutputWriter out = new OutputWriter(outputStream);
		TaskA solver = new TaskA();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskA {
    public void solve(int testNumber, InputReader in, OutputWriter out) {
        int n = in.nextInt();
        int k = in.nextInt();
        if (k == 1) {
            if (n == 1)
                out.println('a');
            else
                out.println("-1");
            return;
        }
        if (k == 2) {
            if (n == 1) {
                out.println("-1");
            } else {
                for (int i = 0; i < n; i++) {
                    out.print(i % 2 == 0 ? 'a' : 'b');
                }
                out.println();
            }
            return;
        }
        if (n < k) {
            out.println("-1");
            return;
        }
        if ((n - k) % 2 == 0) {
            int t = (n - k) / 2;
            for (int i = 0; i < t; i++) {
                out.print("ab");
            }
            for (int i = 0; i < k; i++) {
                out.print((char) (i + 'a'));
            }
            out.println();
        } else {
            int t = (n - k) / 2 + 1;
            for (int i = 0; i < t; i++) {
                out.print("ab");
            }
            for (int i = 0; i < k; i++) {
                if (i != 1)
                    out.print((char) (i + 'a'));
            }
            out.println();
        }

    }
}

class InputReader {

    private InputStream stream;
    private byte[] buf = new byte[1 << 16];
    private int curChar;
    private int numChars;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public int nextInt() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c & 15;
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public static boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    }

class OutputWriter {
    private PrintWriter writer;

    public OutputWriter(OutputStream stream) {
        writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(stream)));
    }

    public OutputWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }

    public void println(char x) {
        writer.println(x);
    }

    public void println() {
        writer.println();
    }

    public void println(String x) {
        writer.println(x);
    }

    public void print(char c) {
        writer.print(c);
    }

    public void close() {
        writer.close();
    }

    public void print(String s) {
        writer.print(s);
    }

    }

