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.*;
import java.math.*;
import static java.lang.Math.*;
import java.security.SecureRandom;
import static java.util.Arrays.*;
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import sun.misc.Regexp;
import java.awt.geom.*;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import sun.net.www.content.text.plain;

public class Main {

    public static void main(String[] args) throws IOException {
        new Main().run();
    }
    StreamTokenizer in;
    PrintWriter out;
//deb////////////////////////////////////////////////

    public static void deb(String n, Object n1) {
        System.out.println(n + " is : " + n1);
    }

    public static void deb(int[] A) {

        for (Object oo : A) {
            System.out.print(oo + " ");
        }
        System.out.println("");
    }

    public static void deb(long[] A) {

        for (Object oo : A) {
            System.out.print(oo + " ");
        }
        System.out.println("");
    }

    public static void deb(boolean[] A) {

        for (Object oo : A) {
            System.out.print(oo + " ");
        }
        System.out.println("");
    }

    public static void deb(double[] A) {

        for (Object oo : A) {
            System.out.print(oo + " ");
        }
        System.out.println("");
    }

    public static void deb(String[] A) {

        for (Object oo : A) {
            System.out.print(oo + " ");
        }
        System.out.println("");
    }

    public static void deb(int[][] A) {
        for (int i = 0; i < A.length; i++) {
            for (Object oo : A[i]) {
                System.out.print(oo + " ");
            }
            System.out.println("");
        }

    }

    public static void deb(double[][] A) {
        for (int i = 0; i < A.length; i++) {
            for (Object oo : A[i]) {
                System.out.print(oo + " ");
            }
            System.out.println("");
        }

    }

    public static void deb(long[][] A) {
        for (int i = 0; i < A.length; i++) {
            for (Object oo : A[i]) {
                System.out.print(oo + " ");
            }
            System.out.println("");
        }

    }

    public static void deb(String[][] A) {
        for (int i = 0; i < A.length; i++) {
            for (Object oo : A[i]) {
                System.out.print(oo + " ");
            }
            System.out.println("");
        }

    }
    /////////////////////////////////////////////////////////////

    int nextInt() throws IOException {
        in.nextToken();
        return (int) in.nval;
    }

    long nextLong() throws IOException {
        in.nextToken();
        return (long) in.nval;
    }

    void run() throws IOException {
//          in = new StreamTokenizer(new BufferedReader(new FileReader("input.txt")));
        // out = new PrintWriter(new FileWriter("output.txt"));
        in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
        out = new PrintWriter(new OutputStreamWriter(System.out));
        solve();
        out.flush();
    }
//boolean inR(int x,int y){
//return (x<=0)&&(x<4)&&(y<=0)&&(y<4);
//}

    @SuppressWarnings("unchecked")
    void solve() throws IOException {
        //   BufferedReader re= new BufferedReader(new FileReader("C:\\Users\\ASELA\\Desktop\\PROBLEMSET\\input\\F\\10.in"));
       //   BufferedReader re = new BufferedReader(new InputStreamReader(System.in));
        //   Scanner sc = new Scanner(System.in);
int N=nextInt(),K=nextInt();
if(K>N)
{
    System.out.println("-1");
    return;
}
StringBuilder sb= new StringBuilder("");
int hh=N-K+2;
     if(K>=2){  
     for (int i = 0; i < hh/2; i++) {
            sb.append("ab");
        }
        if(hh%2==1)sb.append("a");
        for (int i = 2; i < K; i++) {
            sb.append((char)((int)'a'+i));
        }}
     else
     {  
        if(N==1)
             System.out.println("a");
        else
        {
            System.out.println("-1");
        }
     return;
     }
        
       out.println(sb);
    }
}
