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.io.*;
import java.math.BigInteger;

/**
 * @author icemore
 */
public class Main {
    
    
    void solve() throws IOException
    {
        int n, k;
        
        n=nextInt();
        k=nextInt();
        
        if(n<k || k==1 && n>1)
        {
            out.print(-1);
            return;
        }
        
        if(k==1 && n==1)
        {
            out.print('a');
            return;
        }
        
        for (int i = 0; i < n-(k-2); i++) {
            if(i%2==1)
                out.print('b');
            else
                out.print('a');
        }
        
        for (int i = 0; i < k-2; i++) {
            out.print((char)('a'+i+2));
        }
    }
    
    //<editor-fold defaultstate="collapsed" desc="Template">
    
    double nextDouble() throws IOException
    {
        return Double.parseDouble(nextToken());
    }
    
    long nextLong() throws IOException
    {
        return Long.parseLong(nextToken());
    }
    
    int nextInt() throws IOException
    {
        return Integer.parseInt(nextToken());
    }
    
    String nextToken() throws IOException
    {
        while(st==null || !st.hasMoreTokens())
        {
            st = new StringTokenizer(br.readLine());
        }
        
        return st.nextToken();
    }
    
    void run() throws IOException
    {
        boolean onlineJudge = System.getProperty("ONLINE_JUDGE")!=null;
        Reader reader = onlineJudge ? new InputStreamReader(System.in) : new FileReader("input.txt");
        Writer writer = onlineJudge ? new OutputStreamWriter(System.out) : new FileWriter("output.txt");
        
        br = new BufferedReader(reader);
        out = new PrintWriter(writer);
        solve();
        out.close();   
    }
    
    public static void main(String[] args) throws IOException {
        new Main().run();
    }
    
    StringTokenizer st;
    BufferedReader br;
    PrintWriter out;
    //</editor-fold>
}
