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.InputStreamReader;
import java.io. IOException;
public class Main {

    public static void main(String [] args) throws IOException {
        BufferedReader bf =new BufferedReader (new InputStreamReader (System.in));
        String [] s2=bf.readLine().split(" ");
        int n=Integer.parseInt(s2[0]);
        int k=Integer.parseInt(s2[1]);
        int num=k-2;
        
        if((k==1)&&(n==1)){
            System.out.println("a");
            return;
        }
        if(k==1){
            System.out.print("-1");
            return;
        }
        if(n<k){
            System.out.println("-1");
            return;
        }
        int r=0;
        for(int i=0;i<n-num;i++){
            System.out.print((char)('a'+r));
            if(r==1)
                r=0;
                    else
                        r=1;
        }
        for(int i=0;i<num;i++){
            System.out.print((char)('c'+i));
        }
            
    }
}
   