#include <bits/stdc++.h>
using namespace std;
int main() {
  long int i, k, j, l, a, b, len;
  scanf("%ld%ld", &len, &k);
  if (k > len) {
    printf("-1\n");
    return 0;
  }
  if ((len > 1) && (k == 1)) {
    printf("-1\n");
    return 0;
  }
  if ((len == 1) && (k == 1)) {
    printf("a\n");
    return 0;
  }
  if ((len == 2) && (k == 2)) {
    printf("ab\n");
    return 0;
  } else if (k == 2) {
    for (i = 1; i <= len; i++) {
      if (i % 2 == 1)
        printf("a");
      else if (i % 2 == 0)
        printf("b");
    }
    printf("\n");
  } else {
    for (i = 1; i <= (len + 2 - k); i++) {
      if (i % 2 == 1)
        printf("a");
      else if (i % 2 == 0)
        printf("b");
    }
    b = k - 1;
    for (i = 1; i <= (k - 2); i++) {
      a = i % b;
      printf("%c", (98 + a));
    }
    printf("\n");
  }
  return 0;
}

### Prompt

Please provide a cpp coded solution 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;
int main() {
  long int i, k, j, l, a, b, len;
  scanf("%ld%ld", &len, &k);
  if (k > len) {
    printf("-1\n");
    return 0;
  }
  if ((len > 1) && (k == 1)) {
    printf("-1\n");
    return 0;
  }
  if ((len == 1) && (k == 1)) {
    printf("a\n");
    return 0;
  }
  if ((len == 2) && (k == 2)) {
    printf("ab\n");
    return 0;
  } else if (k == 2) {
    for (i = 1; i <= len; i++) {
      if (i % 2 == 1)
        printf("a");
      else if (i % 2 == 0)
        printf("b");
    }
    printf("\n");
  } else {
    for (i = 1; i <= (len + 2 - k); i++) {
      if (i % 2 == 1)
        printf("a");
      else if (i % 2 == 0)
        printf("b");
    }
    b = k - 1;
    for (i = 1; i <= (k - 2); i++) {
      a = i % b;
      printf("%c", (98 + a));
    }
    printf("\n");
  }
  return 0;
}

```