#include <bits/stdc++.h>
const int kMojTest = -1;
struct Exception {
  const char* message;
  const int line;
  Exception(const char* m, int l) : message(m), line(l) {}
};
using namespace std;
const double eps = 1e-9;
const int oo = 0x3f3f3f3f;
const long long ooLL = 0x3f3f3f3f3f3f3f3fLL;
int main() {
  for (int N, K; scanf("%d%d", &N, &K) == 2;) {
    if (K > N)
      printf("-1\n");
    else if (N == 1)
      printf("a\n");
    else if (K == 1)
      printf("-1\n");
    else {
      int M = N - K + 2;
      for (int i = 0; i < M; ++i) printf("%c", 'a' + (i & 1));
      for (int i = M, j = 2; i < N; ++i, ++j) printf("%c", 'a' + j);
      printf("\n");
    }
  }
  return 0;
}

### 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>
const int kMojTest = -1;
struct Exception {
  const char* message;
  const int line;
  Exception(const char* m, int l) : message(m), line(l) {}
};
using namespace std;
const double eps = 1e-9;
const int oo = 0x3f3f3f3f;
const long long ooLL = 0x3f3f3f3f3f3f3f3fLL;
int main() {
  for (int N, K; scanf("%d%d", &N, &K) == 2;) {
    if (K > N)
      printf("-1\n");
    else if (N == 1)
      printf("a\n");
    else if (K == 1)
      printf("-1\n");
    else {
      int M = N - K + 2;
      for (int i = 0; i < M; ++i) printf("%c", 'a' + (i & 1));
      for (int i = M, j = 2; i < N; ++i, ++j) printf("%c", 'a' + j);
      printf("\n");
    }
  }
  return 0;
}

```