#include <bits/stdc++.h>
using namespace std;
void T_T() {
  ios::sync_with_stdio(0);
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
}
int main() {
  T_T;
  char s = 'a';
  bool yes = false;
  int n, m;
  cin >> n >> m;
  if (m > n || m == 1 && n != 1) return puts("-1"), 0;
  while (n--) {
    cout << s;
    if (n - (m - 2) <= 0) {
      if (yes)
        s++;
      else
        s++, s++, yes = true;
    } else if (yes)
      s--, yes = false;
    else
      s++, yes = true;
  }
}

### Prompt

Please provide a CPP coded solution to the problem described below:

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;
void T_T() {
  ios::sync_with_stdio(0);
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
}
int main() {
  T_T;
  char s = 'a';
  bool yes = false;
  int n, m;
  cin >> n >> m;
  if (m > n || m == 1 && n != 1) return puts("-1"), 0;
  while (n--) {
    cout << s;
    if (n - (m - 2) <= 0) {
      if (yes)
        s++;
      else
        s++, s++, yes = true;
    } else if (yes)
      s--, yes = false;
    else
      s++, yes = true;
  }
}

```