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() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int i, j, k, z, x = 1e18, y = -1, sum = 1e18, p, q, l, T, m,
                            ans = 0;
  long long int n;
  double xx, yy;
  bool f = false;
  string s;
  cin >> n >> k;
  if (k == 1) {
    if (n == 1)
      cout << 'a';
    else
      cout << "-1";
  } else if (n >= k) {
    for (i = 1; i <= n - (k - 2); i++) {
      if (i % 2 == 1)
        cout << 'a';
      else
        cout << 'b';
    }
    char c = 'c';
    for (i = 1; i <= k - 2; i++) cout << c++;
  } else
    cout << -1;
  return 0;
}
