#include <bits/stdc++.h>
using namespace std;
const long long int N = 100005;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  {
    long long int i, j, k, n, m, ans = 0, cnt = 0, sum = 0;
    string s, t;
    cin >> n >> k;
    if (k > n || (k == 1 && n > 1)) {
      cout << "-1";
      return 0;
    }
    t = "abcdefghijklmnopqrstuvwxyz";
    m = k - 2;
    j = 2;
    while (m > 0) {
      s += t[j];
      j++;
      m--;
    }
    reverse((s).begin(), (s).end());
    n -= max((long long int)0, (k - 2));
    if (n % 2 != 0) {
      j = 0;
      while (n > 0) {
        if (j % 2 == 0) {
          s += 'a';
        } else {
          s += 'b';
        }
        j++;
        n--;
      }
    } else {
      j = 0;
      while (n > 0) {
        if (j % 2 != 0) {
          s += 'a';
        } else {
          s += 'b';
        }
        j++;
        n--;
      }
    }
    reverse((s).begin(), (s).end());
    cout << s;
  }
}

### Prompt

Develop a solution in cpp to the problem described below:

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 N = 100005;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  {
    long long int i, j, k, n, m, ans = 0, cnt = 0, sum = 0;
    string s, t;
    cin >> n >> k;
    if (k > n || (k == 1 && n > 1)) {
      cout << "-1";
      return 0;
    }
    t = "abcdefghijklmnopqrstuvwxyz";
    m = k - 2;
    j = 2;
    while (m > 0) {
      s += t[j];
      j++;
      m--;
    }
    reverse((s).begin(), (s).end());
    n -= max((long long int)0, (k - 2));
    if (n % 2 != 0) {
      j = 0;
      while (n > 0) {
        if (j % 2 == 0) {
          s += 'a';
        } else {
          s += 'b';
        }
        j++;
        n--;
      }
    } else {
      j = 0;
      while (n > 0) {
        if (j % 2 != 0) {
          s += 'a';
        } else {
          s += 'b';
        }
        j++;
        n--;
      }
    }
    reverse((s).begin(), (s).end());
    cout << s;
  }
}

```