#include <bits/stdc++.h>
using namespace std;
int main() {
  string s[24] = {"c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n",
                  "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"};
  string t[26] = {"a", "b", "c", "d", "e", "f", "g", "h", "i",
                  "j", "k", "l", "m", "n", "o", "p", "q", "r",
                  "s", "t", "u", "v", "w", "x", "y", "z"};
  long long int n, k, flag = 0, i;
  cin >> n >> k;
  string res = "", cut = "";
  if (n < k || (k == 1 && n != 1))
    cout << "-1" << endl;
  else if (n == k) {
    for (i = 0; i < k; i++) {
      res += t[i];
    }
    cout << res << endl;
  } else {
    for (i = k - 2; i >= 1; i--) {
      res += s[i - 1];
      flag++;
    }
    for (i = 1; i <= n - flag; i += 1) {
      if ((i % 2) != 0)
        cut += 'a';
      else
        cut += 'b';
    }
    reverse(res.begin(), res.end());
    cout << cut << res << endl;
  }
  return 0;
}

### Prompt

Construct a CPP code solution to the problem outlined:

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

### Response

```cpp
#include <bits/stdc++.h>
using namespace std;
int main() {
  string s[24] = {"c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n",
                  "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"};
  string t[26] = {"a", "b", "c", "d", "e", "f", "g", "h", "i",
                  "j", "k", "l", "m", "n", "o", "p", "q", "r",
                  "s", "t", "u", "v", "w", "x", "y", "z"};
  long long int n, k, flag = 0, i;
  cin >> n >> k;
  string res = "", cut = "";
  if (n < k || (k == 1 && n != 1))
    cout << "-1" << endl;
  else if (n == k) {
    for (i = 0; i < k; i++) {
      res += t[i];
    }
    cout << res << endl;
  } else {
    for (i = k - 2; i >= 1; i--) {
      res += s[i - 1];
      flag++;
    }
    for (i = 1; i <= n - flag; i += 1) {
      if ((i % 2) != 0)
        cut += 'a';
      else
        cut += 'b';
    }
    reverse(res.begin(), res.end());
    cout << cut << res << endl;
  }
  return 0;
}

```