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.*;

public class A {

	void solve() throws IOException {
		int n = nextInt();
		int k = nextInt();
		if (n < k) {
			out.println("-1");
			return;
		}
		if (k == 1) {
			if (n == 1) {
				out.println("a");
			} else {
				out.println("-1");
			}
			return;
		}
		k-=2;
		for (int i = 0; i < n - k; i++) {
			if (i % 2 == 0) {
				out.print("a");
			} else {
				out.print("b");
			}
		}
		for (int i = n - k; i < n; i++) {
			out.print((char) (i - (n - k) + 'c'));
		}
		out.println();

	}

	void run() throws IOException {
		br = new BufferedReader(new InputStreamReader(System.in));
		out = new PrintWriter(System.out);
		solve();
		out.close();
	}

	public static void main(String[] args) throws IOException {
		new A().run();
	}

	BufferedReader br;
	PrintWriter out;
	StringTokenizer str;

	String next() throws IOException {
		while (str == null || !str.hasMoreTokens()) {
			str = new StringTokenizer(br.readLine());
		}
		return str.nextToken();
	}

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

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

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

}
