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.InputStreamReader;
import java.io.IOException;
import java.util.StringTokenizer;

public class Main {

	public static void main(String[] args) {
		FastReader reader = new FastReader();
		int n = reader.nextInt();
		int k = reader.nextInt();
		reader.close();
		if (k > n || (k < n && k == 1))
			System.out.println(-1);
		else {
			char[] ans = new char[n];
			for (int i = n - 1, j = k - 1; i >= Math.max(0, n - k + 2); --i, --j)
				ans[i] = (char) ('a' + j);
			for (int i = 0, j = 0; i < Math.min(n, n - k + 2); ++i, ++j)
				ans[i] = (char) ('a' + j % 2);
			System.out.println(new String(ans));
		}
	}

	public static void print2D(int[][] mat) {
		StringBuilder sb = new StringBuilder("");
		for (int i = 0; i < mat.length; ++i) {
			for (int j = 0; j < mat[i].length; ++j)
				sb.append(mat[i][j] + " ");
			sb.append("\n");
		}
		System.out.println(sb.toString().trim());
	}

	static class FastReader {
		BufferedReader br;
		StringTokenizer st;

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

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine().trim());
				} 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());
		}

		String nextLine() {
			StringBuilder str = new StringBuilder("");
			try {
				str.append(br.readLine().trim());
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str.toString();
		}

		int[] readArray(int size) {
			int[] ar = new int[size];
			for (int i = 0; i < size; ++i)
				ar[i] = nextInt();
			return ar;
		}

		void close() {
			try {
				br.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}