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.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;


public class z {

    public static void main(String... args)throws  IOException{
        br = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);

        int n = nextInt();
        int k =nextInt();
        if(n<k|| (k==1 && n!=1)){
            out.print(-1);
        }               else
        if(n==k ){

            char c ='a';
            for ( int i=0;i<n;i++ ){
                out.print(c);
                c++;
            }
        }
        else
        {
            k-=2;
            char a = 'c';

            for ( int i =0;i<n-k;i++ ) {

                if(i%2==0) out.print('a');
                else out.print('b');
            }
            while ( k>0 ){
                out.print(a);
                a++;
                k--;
            }
        }
        out.close();
    }



    public static StringTokenizer st;
    public static BufferedReader br;
    public static PrintWriter out;

    public static String nextToken() throws IOException {
        while ( st == null || !st.hasMoreTokens() ) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }

    public static int nextInt() throws NumberFormatException, IOException {
        return Integer.parseInt(nextToken());
    }

    public static double nextDouble() throws NumberFormatException, IOException {
        return Double.parseDouble(nextToken());
    }

    public static long nextLong() throws NumberFormatException, IOException {
        return Long.parseLong(nextToken());
    }

    public static int[] ReadIntArray(int n) throws IOException {
        int[] a = new int[n];
        for ( int i = 0; i < n; i++ ) {
            a[i] = nextInt();
        }
        return a;

    }
    public static void PrintArray(int[] arr) throws IOException{
        for ( int i = 0; i < arr.length; i++ )
            out.print(arr[i] + " ");

    }
    public static long[] ShuffleArray(long [] arr)
    {
        Random rnd = new Random();

        int n = arr.length;
        long    temp;
        int     ind;
        while (n > 1)
        {
            ind = rnd.nextInt(n--);
            temp = arr[ind];
            arr[ind] = arr[n];
            arr[n] = temp;
        }

        return arr;
    }
}