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.util.*;
public class j
{
public static void main(String a[])throws IOException
{
BufferedReader b=new BufferedReader(new InputStreamReader(System.in));
int k=0,i=0,t=0,j=0,n=0,m=0;
String s,r="\0";
s=b.readLine();
StringTokenizer c=new StringTokenizer(s);
m=Integer.parseInt(c.nextToken());
n=Integer.parseInt(c.nextToken());
if(n>m)
{
System.out.print("-1");
System.exit(0);
}
else
{
if(n==1)
{
if(m==1)
{
System.out.print("a");
System.exit(0);
}
else
{
System.out.print("-1");
System.exit(0);
}
}
if(n==2)
{
for(i=1;i<=m;i++)
{
if(i%2==1)
System.out.print("a");
else
System.out.print("b");
}
}
if(n>2)
{
for(i=1;i<=n-2;i++)
r+=(char)(i+98);
r=r.trim();
if(m==n)
{
for(i=1;i<=m;i++)
System.out.print((char)(i+96));
System.exit(0);
}
if(m-n==1)
{
System.out.print("aba");
for(i=1;i<=m-3;i++)
System.out.print((char)(i+98));
System.exit(0);
}
if(m-n>=2)
{
for(i=1;i<=m-(n-2);i++)
{
if(i%2==1)
System.out.print("a");
else
System.out.print("b");
}
System.out.print(r);
}
}
}
}
}