#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long int n, k;
  cin >> n >> k;
  if (k > n)
    cout << -1 << "\n";
  else if (n != 1 && k == 1)
    cout << -1 << "\n";
  else if (k == n) {
    char x = 'a';
    for (long long int i = 1; i <= (long long int)n; i++) {
      cout << x;
      x++;
    }
  } else if (k < n) {
    long long int t = n - (k - 2);
    for (long long int i = 1; i <= (long long int)t; i++) {
      if (i % 2 != 0)
        cout << "a";
      else
        cout << "b";
    }
    long long int q = n - t;
    char y = 'c';
    if (q > 0) {
      for (long long int i = 1; i <= (long long int)q; i++) {
        cout << y;
        y++;
      }
    }
  }
}
int main() {
  std::ios::sync_with_stdio(false);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}

### Prompt

Your task is to create 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;
void solve() {
  long long int n, k;
  cin >> n >> k;
  if (k > n)
    cout << -1 << "\n";
  else if (n != 1 && k == 1)
    cout << -1 << "\n";
  else if (k == n) {
    char x = 'a';
    for (long long int i = 1; i <= (long long int)n; i++) {
      cout << x;
      x++;
    }
  } else if (k < n) {
    long long int t = n - (k - 2);
    for (long long int i = 1; i <= (long long int)t; i++) {
      if (i % 2 != 0)
        cout << "a";
      else
        cout << "b";
    }
    long long int q = n - t;
    char y = 'c';
    if (q > 0) {
      for (long long int i = 1; i <= (long long int)q; i++) {
        cout << y;
        y++;
      }
    }
  }
}
int main() {
  std::ios::sync_with_stdio(false);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}

```