#include <bits/stdc++.h>
using namespace std;
bool notPossible(long long arr[], long long n) {
  if (arr[0] == 0 || arr[n - 1] == 0) return true;
  for (long long i = 0; i < n - 1; i++)
    if (arr[i] == 0 && arr[i + 1] == 0) return true;
  return false;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  long long n, k;
  cin >> n >> k;
  if (k > n || (k == 1 && n > 1))
    cout << "-1";
  else if (n == k) {
    string str;
    for (long long i = 0; i < n; i++) str += ('a' + i);
    cout << str;
  } else {
    bool f = true;
    for (long long i = 0; i < n - k + 2; i++) {
      if (f)
        f = false, cout << "a";
      else
        f = true, cout << "b";
    }
    string str;
    for (long long i = 2; i < k; i++) str += ('a' + i);
    cout << str;
  }
  return 0;
}

### 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;
bool notPossible(long long arr[], long long n) {
  if (arr[0] == 0 || arr[n - 1] == 0) return true;
  for (long long i = 0; i < n - 1; i++)
    if (arr[i] == 0 && arr[i + 1] == 0) return true;
  return false;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  long long n, k;
  cin >> n >> k;
  if (k > n || (k == 1 && n > 1))
    cout << "-1";
  else if (n == k) {
    string str;
    for (long long i = 0; i < n; i++) str += ('a' + i);
    cout << str;
  } else {
    bool f = true;
    for (long long i = 0; i < n - k + 2; i++) {
      if (f)
        f = false, cout << "a";
      else
        f = true, cout << "b";
    }
    string str;
    for (long long i = 2; i < k; i++) str += ('a' + i);
    cout << str;
  }
  return 0;
}

```