#include <bits/stdc++.h>
using namespace std;
long long ar[100005];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  long long t = 1;
  while (t--) {
    int n, k;
    cin >> n >> k;
    if (n == k) {
      char val = 'a';
      for (int i = int(0); i < int(n); i++) cout << val++;
    } else if (k > n)
      cout << -1;
    else {
      if (k == 1)
        cout << -1;
      else {
        cout << "ab";
        int temp = n - k;
        k -= 2;
        if (temp % 2 == 0) {
          temp /= 2;
          while (temp--) cout << "ab";
        } else {
          temp /= 2;
          while (temp--) cout << "ab";
          cout << "a";
        }
        char qw = 'c';
        for (int i = int(0); i < int(k); i++) cout << qw++;
      }
    }
  }
}

### Prompt

Your challenge is to write 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;
long long ar[100005];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  long long t = 1;
  while (t--) {
    int n, k;
    cin >> n >> k;
    if (n == k) {
      char val = 'a';
      for (int i = int(0); i < int(n); i++) cout << val++;
    } else if (k > n)
      cout << -1;
    else {
      if (k == 1)
        cout << -1;
      else {
        cout << "ab";
        int temp = n - k;
        k -= 2;
        if (temp % 2 == 0) {
          temp /= 2;
          while (temp--) cout << "ab";
        } else {
          temp /= 2;
          while (temp--) cout << "ab";
          cout << "a";
        }
        char qw = 'c';
        for (int i = int(0); i < int(k); i++) cout << qw++;
      }
    }
  }
}

```