#include <bits/stdc++.h>
using namespace std;
char j;
int i, n, k, x;
int main() {
  cin >> n >> k;
  if (k <= 2) {
    if (k == 1) {
      if (n == 1) {
        cout << "a";
      } else {
        cout << -1;
      }
    } else {
      if (n == 1) {
        cout << -1;
      }
      if (n == 2) {
        cout << "ab";
      }
      if (n > 2) {
        while (n - 2 >= 0) {
          n = n - 2;
          cout << "ab";
        }
        if (n == 1) {
          cout << "a";
        }
      }
    }
  } else {
    if (n < k) {
      cout << -1;
    } else {
      if (n == k) {
        for (i = 1; i <= n; i++) {
          j = 'a' + (i - 1);
          cout << j;
        }
      } else {
        k = k - 2;
        while (n - 2 >= k) {
          n = n - 2;
          cout << "ab";
        }
        x = n - k;
        for (i = 1; i <= x; i++) {
          if (i % 2 == 1) {
            cout << "a";
          } else {
            cout << "b";
          }
        }
        for (i = 1; i <= k; i++) {
          j = 'b' + i;
          cout << j;
        }
      }
    }
  }
  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;
char j;
int i, n, k, x;
int main() {
  cin >> n >> k;
  if (k <= 2) {
    if (k == 1) {
      if (n == 1) {
        cout << "a";
      } else {
        cout << -1;
      }
    } else {
      if (n == 1) {
        cout << -1;
      }
      if (n == 2) {
        cout << "ab";
      }
      if (n > 2) {
        while (n - 2 >= 0) {
          n = n - 2;
          cout << "ab";
        }
        if (n == 1) {
          cout << "a";
        }
      }
    }
  } else {
    if (n < k) {
      cout << -1;
    } else {
      if (n == k) {
        for (i = 1; i <= n; i++) {
          j = 'a' + (i - 1);
          cout << j;
        }
      } else {
        k = k - 2;
        while (n - 2 >= k) {
          n = n - 2;
          cout << "ab";
        }
        x = n - k;
        for (i = 1; i <= x; i++) {
          if (i % 2 == 1) {
            cout << "a";
          } else {
            cout << "b";
          }
        }
        for (i = 1; i <= k; i++) {
          j = 'b' + i;
          cout << j;
        }
      }
    }
  }
  return 0;
}

```