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.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class C {
    private static BufferedReader in;
    private static StringTokenizer st;
    private static PrintWriter out;
    
    public static void main(String[] args) throws NumberFormatException, IOException {
        in = new BufferedReader(new InputStreamReader(System.in));
        st = new StringTokenizer("");
        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        int n = nextInt();
        int k = nextInt();
        char s [] = new char[n];
        for (int i = 0; i < n; i++) {
            s[i] = 'a';
        }
        if(n==1 && k ==1){
            out.print("a");
            out.close();
            return;
        }
        if(k==1 && n>1){
            out.print(-1);
            out.close();
            return;
        }
        
        if(n<k){
            out.println(-1);
            out.close();
            return;
        }else{
            k-=2;
            int alf = 2;
            for (int i = n - k; i < n; i++) {
                s[i] =(char) (alf++ + 'a');
            }
            for (int i = 0; i < n-k; i++) {
                if(i % 2 ==1){
                    s[i] = 'b';
                }
            }
        }
        for (int i = 0; i < s.length; i++) {
            out.print(s[i]);
        }
        out.close();
    }
    

    static String next() throws IOException{
        while(!st.hasMoreTokens()){
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }
    static int nextInt() throws NumberFormatException, IOException{
        return Integer.parseInt(next());
    }
    static long nextLong() throws NumberFormatException, IOException{
        return Long.parseLong(next());
    }
    
    static double nextDouble() throws NumberFormatException, IOException{
        return Double.parseDouble(next());
    }

}
