#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000005;
const long long INFLL = 1000000000000000002ll;
const long long MOD = 1000000007;
inline long long min(long long a, long long b, long long c) {
  return min(min(a, b), c);
}
inline long long min(long long a, long long b, long long c, long long d) {
  return min(min(min(a, b), c), d);
}
inline long long max(long long a, long long b, long long c) {
  return max(max(a, b), c);
}
inline long long max(long long a, long long b, long long c, long long d) {
  return max(max(max(a, b), c), d);
}
int N, K;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> N >> K;
  if (K == 1) {
    if (N > 1)
      cout << -1;
    else
      cout << "a";
    return 0;
  }
  if (N < K) {
    cout << -1;
    return 0;
  }
  string ans;
  for (int i = (1); i <= (N - K + 2); i++) {
    if (N - i >= K - 2) {
      if (i & 1)
        ans += 'a';
      else
        ans += 'b';
    }
  }
  for (int i = (3); i <= (K); i++) ans += (char)('a' + i - 1);
  cout << ans;
}

### Prompt

Your challenge is to write 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;
const int INF = 1000000005;
const long long INFLL = 1000000000000000002ll;
const long long MOD = 1000000007;
inline long long min(long long a, long long b, long long c) {
  return min(min(a, b), c);
}
inline long long min(long long a, long long b, long long c, long long d) {
  return min(min(min(a, b), c), d);
}
inline long long max(long long a, long long b, long long c) {
  return max(max(a, b), c);
}
inline long long max(long long a, long long b, long long c, long long d) {
  return max(max(max(a, b), c), d);
}
int N, K;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> N >> K;
  if (K == 1) {
    if (N > 1)
      cout << -1;
    else
      cout << "a";
    return 0;
  }
  if (N < K) {
    cout << -1;
    return 0;
  }
  string ans;
  for (int i = (1); i <= (N - K + 2); i++) {
    if (N - i >= K - 2) {
      if (i & 1)
        ans += 'a';
      else
        ans += 'b';
    }
  }
  for (int i = (3); i <= (K); i++) ans += (char)('a' + i - 1);
  cout << ans;
}

```