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 {
    public static void main(String[] args) throws Exception {
        new A().solve();
        // new FileInputStream(new File("input.txt")),
        // new PrintStream(new FileOutputStream(new File("output.txt"))));
    }

    void solve() throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);
        String[] sp;

        int n = sc.nextInt();
        int k = sc.nextInt();
        if (n < k) {
            System.out.println("-1");
            return;
        }
        if (k == 1) {
            if (n == 1) {
                System.out.println("a");
            } else {
                System.out.println("-1");
            }
            return;
        }

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n - (k - 2); i++) {
            sb.append((char) ('a' + i % 2));
        }
        for (int i = n - (k - 2); i < n; i++) {
            sb.append((char) ('c' + (i - (n - (k - 2)))));
        }
        System.out.println(sb);
    }
}
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
//
