#include <bits/stdc++.h>
using namespace std;
char s[1000005];
int main() {
  int n, k;
  while (scanf("%d%d", &n, &k) != EOF) {
    if (k > n || (n > 1 && k == 1)) {
      printf("-1\n");
      break;
    } else {
      if ((n - k) % 2) {
        for (int i = 1; i <= n - k + 1; i++) {
          s[i] = 'a';
          s[i + 1] = 'b';
          i++;
        }
        s[n - k + 2] = 'a';
        int c = 99;
        for (int i = n - k + 3; i <= n; i++) {
          s[i] = c;
          c++;
        }
      } else {
        for (int i = 1; i <= n - k; i++) {
          s[i] = 'a';
          s[i + 1] = 'b';
          i++;
        }
        int c = 97;
        for (int i = n - k + 1; i <= n; i++) {
          s[i] = c;
          c++;
        }
      }
    }
    for (int i = 1; i <= n; i++) printf("%c", s[i]);
    cout << endl;
  }
  return 0;
}

### Prompt

Please formulate 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;
char s[1000005];
int main() {
  int n, k;
  while (scanf("%d%d", &n, &k) != EOF) {
    if (k > n || (n > 1 && k == 1)) {
      printf("-1\n");
      break;
    } else {
      if ((n - k) % 2) {
        for (int i = 1; i <= n - k + 1; i++) {
          s[i] = 'a';
          s[i + 1] = 'b';
          i++;
        }
        s[n - k + 2] = 'a';
        int c = 99;
        for (int i = n - k + 3; i <= n; i++) {
          s[i] = c;
          c++;
        }
      } else {
        for (int i = 1; i <= n - k; i++) {
          s[i] = 'a';
          s[i + 1] = 'b';
          i++;
        }
        int c = 97;
        for (int i = n - k + 1; i <= n; i++) {
          s[i] = c;
          c++;
        }
      }
    }
    for (int i = 1; i <= n; i++) printf("%c", s[i]);
    cout << endl;
  }
  return 0;
}

```