#include <bits/stdc++.h>
using namespace std;
string s;
bool u[200];
int main() {
  long long n, k;
  cin >> n >> k;
  if (k > n)
    cout << -1 << endl;
  else if (k == 1 && n > 1)
    cout << -1 << endl;
  else {
    s = "";
    memset(u, 0, sizeof u);
    for (int i = 0; i < n; i++) {
      if (i == 0) {
        s += 'a';
        u[0] = 1;
      } else {
        for (int j = 0; j < 26; j++)
          if (s[i - 1] - 'a' != j) {
            s += 'a' + j;
            u[j] = 1;
            j = 27;
          }
      }
    }
    long long c = 0;
    for (int i = 0; i < 200; i++)
      if (u[i]) c++;
    long long d = k - c;
    for (int i = s.length() - d; i < s.length(); i++) {
      for (int j = 0; j < 26; j++)
        if (!u[j]) {
          s[i] = 'a' + j;
          u[j] = 1;
          j = 27;
        }
    }
    cout << s << endl;
  }
  return 0;
}

### Prompt

Please formulate 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;
string s;
bool u[200];
int main() {
  long long n, k;
  cin >> n >> k;
  if (k > n)
    cout << -1 << endl;
  else if (k == 1 && n > 1)
    cout << -1 << endl;
  else {
    s = "";
    memset(u, 0, sizeof u);
    for (int i = 0; i < n; i++) {
      if (i == 0) {
        s += 'a';
        u[0] = 1;
      } else {
        for (int j = 0; j < 26; j++)
          if (s[i - 1] - 'a' != j) {
            s += 'a' + j;
            u[j] = 1;
            j = 27;
          }
      }
    }
    long long c = 0;
    for (int i = 0; i < 200; i++)
      if (u[i]) c++;
    long long d = k - c;
    for (int i = s.length() - d; i < s.length(); i++) {
      for (int j = 0; j < 26; j++)
        if (!u[j]) {
          s[i] = 'a' + j;
          u[j] = 1;
          j = 27;
        }
    }
    cout << s << endl;
  }
  return 0;
}

```