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;
  string s = "";
  if (k > n) {
    cout << -1;
    return 0;
  } else if (k == 1 && n > 1) {
    cout << -1;
    return 0;
  } else if (k == 1 && n == 1) {
    cout << "a";
    return 0;
  } else if (k <= n) {
    int t = n - k + 2;
    if (t % 2 == 0) {
      int c = t / 2;
      while (c--) {
        s += "ab";
      }
    } else {
      int c = (t - 1) / 2;
      while (c--) {
        s += "ab";
      }
      s += "a";
    }
    int z = k - 2;
    int v = 2;
    while (z--) {
      s += 'a' + v;
      v++;
    }
    cout << s;
  }
}
