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.util.*;
import java.util.Map.Entry;
import java.io.*;
import java.awt.Point;
import java.math.BigInteger;

import static java.lang.Math.*;

public class Codeforces_Solutions implements Runnable{
    
    final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
    
    BufferedReader in;
    PrintWriter out;
    StringTokenizer tok = new StringTokenizer("");
    
    void init() throws FileNotFoundException{
        if (ONLINE_JUDGE){
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);
        }else{
            in = new BufferedReader(new FileReader("input.txt"));
            out = new PrintWriter("output.txt");
        }
    }

    String readString() throws IOException{
        while(!tok.hasMoreTokens()){
            try{
                tok = new StringTokenizer(in.readLine());
            }catch (Exception e){
                return null;
            }
        }
        return tok.nextToken();
    }
    
    int readInt() throws IOException{
        return Integer.parseInt(readString());
    }
    
    long readLong() throws IOException{
        return Long.parseLong(readString());
    }
    
    double readDouble() throws IOException{
        return Double.parseDouble(readString());
    }
    
    public static void main(String[] args){
        new Thread(null, new Codeforces_Solutions(), "", 128 * (1L << 20)).start();
    }
    
    long timeBegin, timeEnd;

    void time(){
        timeEnd = System.currentTimeMillis();
        System.err.println("Time = " + (timeEnd - timeBegin));
    }
    
    long memoryTotal, memoryFree;
    

    void memory(){
        memoryFree = Runtime.getRuntime().freeMemory();
        System.err.println("Memory = " + ((memoryTotal - memoryFree) >> 10) + " KB");
    }
    
    void debug(Object... objects){
        if (DEBUG){
            for (Object o: objects){
                System.err.println(o.toString());
            }
        }
    }
    
    public void run(){
        try{
            timeBegin = System.currentTimeMillis();
            memoryTotal = Runtime.getRuntime().freeMemory();
            init();
            solve();
            out.close();
            time();
            memory();
        }catch (Exception e){
            e.printStackTrace(System.err);
            System.exit(-1);
        }
    }
    
    
    boolean DEBUG = false;
    
    class Point {
        int x;
        int y;
        
        public Point(int x, int y) {
            this.x=x;
            this.y=y;
        }
        
    }
    
    
    void solve() throws IOException{
        
         int n = readInt();
         int k = readInt();
         if (n<k) {
             out.println(-1);
             return;
         }
         
         if (k==1 && n>1) {
             out.println(-1);
             return;
         }
         
         if (k==1 && n==1) {
             out.println('a');
             return;
         }
         
         char[] a = new char[k]; 
         
         for (int i=0; i<k; i++) a[i]=(char)('a'+i);
         for (int i=0; i<n-k+2; i++) {
             out.print(a[i%2]);
         }
         int j=2;
         for (int i=n-k+2; i<n; i++) {
             
             out.print(a[j++]);
         }
             
    }        
}