#include <bits/stdc++.h>
using namespace std;
const int inf = 2147483647;
const int N = 1e6 + 100;
int getint();
char s[N];
int main() {
  int n = getint(), k = getint(), i;
  if (k > n || (k == 1 && n != 1)) {
    printf("-1");
    return 0;
  }
  while (k > 2) s[n--] = 96 + k--;
  for (i = 1; i <= n; i++) s[i] = i & 1 ? 'a' : 'b';
  puts(s + 1);
  return 0;
}
int getint() {
  int w = 0;
  bool q = 1;
  char c = getchar();
  while ((c < '0' || c > '9') && c != '-') c = getchar();
  if (c == '-') q = 0, c = getchar();
  while (c >= '0' && c <= '9') w = w * 10 + c - '0', c = getchar();
  return q ? w : -w;
}

### Prompt

Create a solution in CPP for 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;
const int inf = 2147483647;
const int N = 1e6 + 100;
int getint();
char s[N];
int main() {
  int n = getint(), k = getint(), i;
  if (k > n || (k == 1 && n != 1)) {
    printf("-1");
    return 0;
  }
  while (k > 2) s[n--] = 96 + k--;
  for (i = 1; i <= n; i++) s[i] = i & 1 ? 'a' : 'b';
  puts(s + 1);
  return 0;
}
int getint() {
  int w = 0;
  bool q = 1;
  char c = getchar();
  while ((c < '0' || c > '9') && c != '-') c = getchar();
  if (c == '-') q = 0, c = getchar();
  while (c >= '0' && c <= '9') w = w * 10 + c - '0', c = getchar();
  return q ? w : -w;
}

```