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 SolutionC {
   BufferedReader in;
   StringTokenizer str;
   PrintWriter out;
   String SK;
   
   String next() throws IOException {
      while ((str == null) || (!str.hasMoreTokens())) {
      SK = in.readLine();
      if (SK == null)
      return null;
      str = new StringTokenizer(SK);
      }
      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());
   }
   char[] charArray() throws IOException{
      return next().toCharArray();
   }
   public static void main(String[] args) throws IOException {
      new SolutionC().run();
   }
   void run() throws IOException {
       in = new BufferedReader(new InputStreamReader(System.in));
      out = new PrintWriter(System.out);
      //in = new BufferedReader(new FileReader("input.txt"));
      //out = new PrintWriter("output.txt");
      solve();
      out.close();
   }
   void solve() throws IOException {
      int n=nextInt();
      int k=nextInt();
      char ch[]=new char[n];

      if(n<k){
         out.println(-1);
         return;
      }
      if(k==1){
         if(n==1){
            out.println("a");
            return;
         }
         out.println(-1);
         return;
      }

      for(int i=0;i<n-(k-2);i++){
         if(i%2==0)
            ch[i]='a';
         else ch[i]='b';
      }
      int s=n-(k-2);
      for (int i=s;i<s+k-2;i++) {
         ch[i]=(char)((i-s)+2+'a');
      }
      out.println(String.valueOf(ch));
   }

}
