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;
int main() {
  int n, k;
  cin >> n >> k;
  if ((n > 1 && k == 1) || (n < k)) {
    cout << -1;
    return 0;
  }
  if (n == 1 && k == 1) {
    cout << "a\n";
    return 0;
  }
  int l = 1;
  int i;
  string s = "";
  string a = "a";
  string b = "b";
  for (i = 0; i < n - (k - 2); i++) {
    if (l == 1) {
      s += a.substr(0, 1);
    } else {
      s += b.substr(0, 1);
    }
    l = l * (-1);
  }
  int j;
  for (i = 'c'; j < k - 2; j++, i++) s += ((char)(i));
  cout << s << "\n";
  return 0;
}
