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 dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
const int mod = 1000000007;
const double pi = (double)22 / 7;
int main() {
  cin.tie(0);
  std::ios::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  if (k > n || (k == 1 && n != 1)) return cout << -1, 0;
  string ans = "";
  n -= k;
  bool flag = 0;
  while (n > 0) {
    if (n > 1) {
      cout << "ab";
      n -= 2;
    } else {
      cout << 'a';
      n--;
      flag = 1;
    }
  }
  if (flag) {
    if (k > 1) {
      ans += "ba";
      k -= 2;
      int c = 2;
      for (int i = 0; i < k; i++, c++) ans += char('a' + c);
    } else {
      ans += 'a';
      k--;
    }
  } else {
    for (int i = 0; i < k; i++) ans += char('a' + i);
  }
  cout << ans;
}
