#include <bits/stdc++.h>
using namespace std;
bool secdes(const pair<long long, long long>& a,
            const pair<long long, long long>& b) {
  if (a.second == b.second) {
    return a.first > b.first;
  } else {
    return a.second > b.second;
  }
}
bool sir(const pair<long long, long long>& a,
         const pair<long long, long long>& b) {
  if (a.first == b.first) {
    return a.second > b.second;
  } else {
    return a.first > b.first;
  }
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, k;
  cin >> n >> k;
  if (n < k)
    return cout << "-1", 0;
  else {
    if (n == 1)
      cout << "a";
    else if (n == 2) {
      if (k == 1)
        return cout << "-1", 0;
      else
        return cout << "ab", 0;
    } else {
      if (k == 1)
        return cout << "-1", 0;
      else {
        long long f = 1;
        for (long long i = 0; i <= n - k + 1; i++) {
          if (f == 1)
            cout << "a";
          else if (f == 2)
            cout << "b";
          if (f == 1)
            f = 2;
          else
            f = 1;
        }
        for (long long i = 2; i < k; i++) cout << (char)(i + 97);
      }
    }
  }
}

### 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>
using namespace std;
bool secdes(const pair<long long, long long>& a,
            const pair<long long, long long>& b) {
  if (a.second == b.second) {
    return a.first > b.first;
  } else {
    return a.second > b.second;
  }
}
bool sir(const pair<long long, long long>& a,
         const pair<long long, long long>& b) {
  if (a.first == b.first) {
    return a.second > b.second;
  } else {
    return a.first > b.first;
  }
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, k;
  cin >> n >> k;
  if (n < k)
    return cout << "-1", 0;
  else {
    if (n == 1)
      cout << "a";
    else if (n == 2) {
      if (k == 1)
        return cout << "-1", 0;
      else
        return cout << "ab", 0;
    } else {
      if (k == 1)
        return cout << "-1", 0;
      else {
        long long f = 1;
        for (long long i = 0; i <= n - k + 1; i++) {
          if (f == 1)
            cout << "a";
          else if (f == 2)
            cout << "b";
          if (f == 1)
            f = 2;
          else
            f = 1;
        }
        for (long long i = 2; i < k; i++) cout << (char)(i + 97);
      }
    }
  }
}

```