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

//@SuppressWarnings("unused")
public class A {

	private final static boolean autoflush = false;

	public A () {
		int N = sc.nextInt();
		int K = sc.nextInt();

		if (N == 1 && K == 1)
			exit("a");

		if (K == 1)
			exit(-1);

		if (K > N)
			exit(-1);

		char [] C = new char [N];
		char L = (char)('c' + (K-2)); int c = N-1;
		for (int q : sep('c', L))
			C[c--] = (char)q;
		for (int i : rep(N))
			if (C[i] == 0)
				if (i % 2 == 0)
					C[i] = 'a';
				else
					C[i] = 'b';

		String res = new String(C);
		exit(res);
	}

	////////////////////////////////////////////////////////////////////////////////////
	private static int [] rep(int N) { return rep(0, N); }
	private static int [] rep(int S, int T) { int [] res = new int [max(T-S, 0)]; for (int i = S; i < T; ++i) res[i-S] = i; return res; }
	private static int [] sep(int S, int T) { int [] res = new int [max(T-S, 0)]; for (int i = S; i < T; ++i) res[T-1-i] = i; return res; }
	private static <T extends Comparable<T>> T max(T x, T y) {
		return x.compareTo(y) > 0 ? x : y;
	}
	////////////////////////////////////////////////////////////////////////////////////
	private final static MyScanner sc = new MyScanner();
	private static class MyScanner {
		private String next() {
			newLine();
			return line[index++];
		}
		private int nextInt() {
			return Integer.parseInt(next());
		}
		//////////////////////////////////////////////
		private boolean eol() {
			return index == line.length;
		}
		private String readLine() {
			try {
				return r.readLine();
			} catch (Exception e) {
				throw new Error (e);
			}
		}
		private final java.io.BufferedReader r;
		private MyScanner () {
			this(new java.io.BufferedReader(new java.io.InputStreamReader(System.in)));
		}
		private MyScanner (java.io.BufferedReader r) {
			try {
				this.r = r;
				while (!r.ready())
					Thread.sleep(1);
				start();
			} catch (Exception e) {
				throw new Error(e);
			}
		}
		private String [] line;
		private int index;
		private void newLine() {
			if (line == null || eol()) {
				line = readLine().split(" ");
				index = 0;
			}
		}
	}
	private static void print (Object o, Object... a) {
		printDelim(" ", o, a);
	}
	private static void printDelim (String delim, Object o, Object... a) {
		pw.println(build(delim, o, a));
	}
	private static void exit (Object o, Object... a) {
		print(o, a);
		exit();
	}
	private static void exit() {
		pw.close();
		System.out.flush();
		System.err.println("------------------");
		System.err.println("Time: " + ((millis() - t) / 1000.0));
		System.exit(0);
	}
	////////////////////////////////////////////////////////////////////////////////////
	private static String build (String delim, Object o, Object... a) {
		StringBuilder b = new StringBuilder();
		append(b, o, delim);
		for (Object p : a)
			append(b, p, delim);
		return b.toString().trim();
	}
	private static void append(StringBuilder b, Object o, String delim) {
		if (o.getClass().isArray()) {
			int L = java.lang.reflect.Array.getLength(o);
			for (int i : rep(L))
				append(b, java.lang.reflect.Array.get(o, i), delim);
		} else if (o instanceof Iterable<?>)
			for (Object p : (Iterable<?>)o)
				append(b, p, delim);
		else
			b.append(delim).append(o);
	}
	////////////////////////////////////////////////////////////////////////////////////
	private static void start() {
		t = millis();
	}
	private static java.io.PrintWriter pw = new java.io.PrintWriter(System.out, autoflush);
	private static long t;
	private static long millis() {
		return System.currentTimeMillis();
	}
	public static void main (String[] args) {
		new A();
		exit();
	}
}
