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.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;

public class C {

	FastScanner in;
	PrintWriter out;

	public void solve() throws IOException {
		int n = in.nextInt();
		int m = in.nextInt();
		if (n < m || (m == 1 && n != 1))
			out.print(new Integer(-1).toString());
		else {
			if (m == 1 && n == 1)
				out.print("a");
			else {
				StringBuilder res = new StringBuilder();
				int pos = 0;
				while (n > m - 2) {
					res.append( (pos % 2) == 0 ? 'a' : 'b');
					++pos;
					--n;
				}
				pos = 0;
				while (n > 0) {
					char a = (char)('c' + pos);
					++pos;
					--n;
					res.append(a);
				}
				out.print(res.toString());
			}
			
		}
	}

	public void run() {
		try {
			in = new FastScanner(System.in);
			out = new PrintWriter(System.out);
			solve();
			out.close();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	class FastScanner {

		BufferedReader br;
		StringTokenizer st;

		FastScanner(InputStream is) {
			br = new BufferedReader(new InputStreamReader(is));
		}

		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());
		}

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

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

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