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>
using namespace std;
const int maxn = 1e6 + 5;
int n, k;
char s[maxn];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> k;
  if ((k > n) || (k == 1 && n >= 2)) {
    puts("-1");
    return 0;
  }
  if (n == 1) {
    puts("a");
    return 0;
  }
  for (int i = n - k + 2, j = 2; i < n; i++, j++) s[i] = 'a' + j;
  for (int i = 0; i < n - k + 2; i++) {
    if (i & 1)
      s[i] = 'b';
    else
      s[i] = 'a';
  }
  puts(s);
  return 0;
}
