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.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;


public class  C{

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Parser p = new Parser(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		int n = p.nextInt();
		int k = p.nextInt();
		if(n<k || (k == 1 && n > 1)){
			pw.print(-1);
			pw.close();
			return;
		}else if(k == 1 && n ==1){
			pw.print("a");
			pw.close();
			return;
		}
		
		for(int i=0; i<n-k+2; ++i){
			if(i%2 == 0){
				pw.print("a");
			}else{
				pw.print("b");
			}
		}
		
		
		for(char i='c'; i<'c'+k-2; ++i){
			pw.print(i);
		}
		
		pw.close();
	}

	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	static class Parser{
		
		StringTokenizer st;
		BufferedReader br;

		public Parser(InputStream is){
			this.br = new BufferedReader( new InputStreamReader(is));
			
		}
		
		public int nextInt(){
			return Integer.parseInt(nextToken());
		}
		
		public double nextDouble(){
			return Double.parseDouble(nextToken());
		}
		
		public String nextString(){
			return nextToken();
		}
		
		public int[] nextIntArray(int s){
			int[] a = new int[s];
			for(int i=0; i<s; ++i){
				a[i] = nextInt();
			}
			return a;
		}
		
		public int[][] nextIntTable(int r, int c){
			int[][] a = new int[r][c];
			for(int i=0; i<r; ++i){
				a[i] = nextIntArray(c);
			}
			return a;
		}
	
		private String nextToken() {
			if( st == null || ! st.hasMoreTokens() ){
				try{
					st = new StringTokenizer( br.readLine());
				}catch( Exception e){
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}
		
	}
}



