#include <bits/stdc++.h>
using namespace std;
const long long int MOD = 1e9 + 7;
const long long int MAXN = 1e6 + 1;
const long double PI = acos(-1);
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    int n, k;
    cin >> n >> k;
    ;
    string ans = "";
    if (k > n || (k == 1 && n != 1))
      cout << -1;
    else {
      bool flag = 0;
      for (int i = 0; i < (n); ++i) {
        if (n - i > k - 2) {
          char x = 'a' + flag;
          ans.push_back(x);
          flag ^= 1;
        } else {
          char x = 'c' + i - (n - k + 2);
          ans.push_back(x);
        }
      }
      cout << ans;
    }
  }
  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;
const long long int MOD = 1e9 + 7;
const long long int MAXN = 1e6 + 1;
const long double PI = acos(-1);
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    int n, k;
    cin >> n >> k;
    ;
    string ans = "";
    if (k > n || (k == 1 && n != 1))
      cout << -1;
    else {
      bool flag = 0;
      for (int i = 0; i < (n); ++i) {
        if (n - i > k - 2) {
          char x = 'a' + flag;
          ans.push_back(x);
          flag ^= 1;
        } else {
          char x = 'c' + i - (n - k + 2);
          ans.push_back(x);
        }
      }
      cout << ans;
    }
  }
  return 0;
}

```