#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  int n, k;
  cin >> n >> k;
  if (n < k or k > 26 or (n > 1 and k == 1))
    cout << "-1" << endl;
  else if (k == 1) {
    bool a = true;
    for (int i = 0; i < n; ++i) {
      if (a)
        cout << 'a';
      else
        cout << 'b';
      a = !a;
    }
    cout << endl;
  } else {
    bool a = true;
    int i = n;
    while (i > k - 2) {
      if (a) {
        s += 'a';
        a = !a;
      } else {
        s += 'b';
        a = !a;
      }
      --i;
    }
    char c = 'c';
    while (i > 0) {
      s += c;
      ++c;
      --i;
    }
    cout << s << endl;
  }
}

### Prompt

Create a solution in CPP for 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 main() {
  string s;
  int n, k;
  cin >> n >> k;
  if (n < k or k > 26 or (n > 1 and k == 1))
    cout << "-1" << endl;
  else if (k == 1) {
    bool a = true;
    for (int i = 0; i < n; ++i) {
      if (a)
        cout << 'a';
      else
        cout << 'b';
      a = !a;
    }
    cout << endl;
  } else {
    bool a = true;
    int i = n;
    while (i > k - 2) {
      if (a) {
        s += 'a';
        a = !a;
      } else {
        s += 'b';
        a = !a;
      }
      --i;
    }
    char c = 'c';
    while (i > 0) {
      s += c;
      ++c;
      --i;
    }
    cout << s << endl;
  }
}

```