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
#include <bits/stdc++.h>
int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  int b = m - 2, a = n - b;
  char prev;
  if (n == m) {
    char j = 'a';
    while (n) {
      printf("%c", j);
      j++;
      n--;
    }
  } else if (m < 2 || m > n)
    printf("-1\n");
  else {
    if (a & 1) {
      printf("a");
      prev = 'a';
      a--;
      while (a) {
        printf("ba");
        prev = 'a';
        a -= 2;
      }
    } else {
      while (a) {
        printf("ab");
        prev = 'b';
        a -= 2;
      }
    }
    char i = 'b';
    while (b > 0) {
      i++;
      printf("%c", i);
      b--;
    }
  }
  printf("\n");
  return 0;
}
