#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:1024000000,1024000000")
const double pi = 2.0 * asin(1.0), eps = 1e-12;
const int maxn = 1100, maxm = 1100000, inf = 0x3f3f3f3f;
int main() {
  int i, j, n, m;
  while (scanf("%d%d", &n, &m) == 2) {
    if (n == m && n == 1) {
      cout << "a" << endl;
      continue;
    }
    if (n < m) {
      puts("-1");
    } else if (n > 1 && m == 1) {
      puts("-1");
    } else {
      for (i = 0; i < n - m + 2; i++) {
        if (i % 2 == 0)
          putchar('a');
        else
          putchar('b');
      }
      for (j = 2; j < m; j++) {
        putchar(j + 'a');
      }
      printf("\n");
    }
  }
  return 0;
}

### Prompt

In Cpp, your task is to solve 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;
#pragma comment(linker, "/STACK:1024000000,1024000000")
const double pi = 2.0 * asin(1.0), eps = 1e-12;
const int maxn = 1100, maxm = 1100000, inf = 0x3f3f3f3f;
int main() {
  int i, j, n, m;
  while (scanf("%d%d", &n, &m) == 2) {
    if (n == m && n == 1) {
      cout << "a" << endl;
      continue;
    }
    if (n < m) {
      puts("-1");
    } else if (n > 1 && m == 1) {
      puts("-1");
    } else {
      for (i = 0; i < n - m + 2; i++) {
        if (i % 2 == 0)
          putchar('a');
        else
          putchar('b');
      }
      for (j = 2; j < m; j++) {
        putchar(j + 'a');
      }
      printf("\n");
    }
  }
  return 0;
}

```