#include <bits/stdc++.h>
using namespace std;
int mat[100 + 5][100 + 5];
int main() {
  int n, k;
  cin >> n >> k;
  if (n < k)
    cout << "-1";
  else {
    if (k == 1) {
      if (n == 1)
        cout << "a";
      else
        cout << "-1";
    } else {
      string s = "";
      int r = n - k;
      while (r) {
        if (r >= 2)
          r -= 2, s.push_back('a'), s.push_back('b');
        else
          r--, s.push_back('a');
      }
      for (int i = 0; i < k; i++) {
        s += ('a' + i);
      }
      for (int i = 1; i < s.size(); i++) {
        if (s[i] == 'a' && s[i - 1] == 'a') swap(s[i], s[i + 1]);
      }
      cout << s;
    }
  }
}

### Prompt

Generate a cpp solution to the following problem:

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 mat[100 + 5][100 + 5];
int main() {
  int n, k;
  cin >> n >> k;
  if (n < k)
    cout << "-1";
  else {
    if (k == 1) {
      if (n == 1)
        cout << "a";
      else
        cout << "-1";
    } else {
      string s = "";
      int r = n - k;
      while (r) {
        if (r >= 2)
          r -= 2, s.push_back('a'), s.push_back('b');
        else
          r--, s.push_back('a');
      }
      for (int i = 0; i < k; i++) {
        s += ('a' + i);
      }
      for (int i = 1; i < s.size(); i++) {
        if (s[i] == 'a' && s[i - 1] == 'a') swap(s[i], s[i + 1]);
      }
      cout << s;
    }
  }
}

```