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;
import java.util.StringTokenizer;

public class PoloThePenguinAndStrings {
    public static void main(String[] args) throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer t = new StringTokenizer(br.readLine());
        int n = Integer.parseInt(t.nextToken());
        int k = Integer.parseInt(t.nextToken());
        if(k>n||(n>1&&k==1)){
            System.out.println(-1);
            return;
        }
        
        StringBuilder prefix =new StringBuilder();
        int count =3;
        while(count<=k){
            char c = (char)(count+96);
            prefix.append(c);
            count++;
        }
        n = n-prefix.length();
        StringBuilder start = new StringBuilder();
        if (n%2==0){
            for (int i =1;i<=n; ){  
                start.append("ab");
                i+=2;
            }
        }
        else{
            for (int i =1;i<=n-1; ){    
                start.append("ab");
                i+=2;
            }
            start.append("a");
        }
        start.append(prefix);
        System.out.println(start);
    }
}