#include <bits/stdc++.h>
using namespace std;
vector<int> v;
bool sol(int i, int s, int n) {
  if (i == n) {
    if (s % 360 == 0)
      return 1;
    else
      return 0;
  }
  return sol(i + 1, s + v[i], n) || sol(i + 1, s - v[i], n);
}
int main() {
  int n, k;
  char c = 'a', f = 'c';
  cin >> n >> k;
  string s;
  if (n < k || k == 1 && n != 1) {
    cout << -1 << endl;
    return 0;
  }
  while (s.size() < n) {
    if (s.size() < n - k + 2) {
      if (c == 'a') {
        s.push_back(c);
        c = 'b';
      } else if (c == 'b') {
        s.push_back(c);
        c = 'a';
      }
    } else {
      s.push_back(f);
      f++;
    }
  }
  cout << s << 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;
vector<int> v;
bool sol(int i, int s, int n) {
  if (i == n) {
    if (s % 360 == 0)
      return 1;
    else
      return 0;
  }
  return sol(i + 1, s + v[i], n) || sol(i + 1, s - v[i], n);
}
int main() {
  int n, k;
  char c = 'a', f = 'c';
  cin >> n >> k;
  string s;
  if (n < k || k == 1 && n != 1) {
    cout << -1 << endl;
    return 0;
  }
  while (s.size() < n) {
    if (s.size() < n - k + 2) {
      if (c == 'a') {
        s.push_back(c);
        c = 'b';
      } else if (c == 'b') {
        s.push_back(c);
        c = 'a';
      }
    } else {
      s.push_back(f);
      f++;
    }
  }
  cout << s << endl;
  return 0;
}

```