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.StringTokenizer;

public class C {
  public static void main(String[] args){
    FastScanner sc = new FastScanner();
    int n = sc.nextInt();
    int k = sc.nextInt();

    if(k > n || (k == 1 && n > 1)) {
      System.out.print(-1);
      return;
    }

    if(n == 1 && k == 1) {
      System.out.print('a');
      return;
    }

    int headerLength = n - k + 2;
    String header = buildHeader(headerLength);
    String tail = buildTail(n - headerLength, 'c');
    System.out.print(header + tail);
  }

  private static String buildHeader(int length) {
    StringBuilder sb = new StringBuilder();
    for(int i = 0; i < length; i++) {
      if(i % 2 == 0) {
        sb.append('a');
      } else {
        sb.append('b');
      }
    }
    return sb.toString();
  }

  private static String buildTail(int length, char start) {
    StringBuilder sb = new StringBuilder();
    for(int i = 0; i < length; i++) {
      sb.append(start++);
    }
    return sb.toString();
  }

  public static class FastScanner {
    BufferedReader br;
    StringTokenizer st;

    public FastScanner(String s) {
      try {
        br = new BufferedReader(new FileReader(s));
      } catch (FileNotFoundException e) {
        e.printStackTrace();
      }
    }

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

    String nextToken() {
      while (st == null || !st.hasMoreElements()) try {
        st = new StringTokenizer(br.readLine());
      } catch (IOException e) {
        e.printStackTrace();
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(nextToken());
    }

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

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