#include <bits/stdc++.h>
using namespace std;
template <class T1>
void debug(T1 e) {
  cout << e << endl;
}
template <class T1, class T2>
void debug(T1 e1, T2 e2) {
  cout << e1 << "\t" << e2 << endl;
}
template <class T1, class T2, class T3>
void debug(T1 e1, T2 e2, T3 e3) {
  cout << e1 << "\t" << e2 << "\t" << e3 << endl;
}
template <class T1, class T2, class T3, class T4>
void debug(T1 e1, T2 e2, T3 e3, T4 e4) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << endl;
}
template <class T1, class T2, class T3, class T4, class T5>
void debug(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << "\t" << e5 << endl;
}
template <class T1, class T2, class T3, class T4, class T5, class T6>
void debug(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5, T6 e6) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << "\t" << e5 << "\t"
       << e6 << endl;
}
int main() {
  int n, k;
  while (cin >> n >> k) {
    if (k > n)
      cout << -1 << endl;
    else if (k == 1 && n > 1)
      cout << "-1\n";
    else {
      int i;
      int hi = n - (k - 2);
      if (hi > n) hi = n;
      for (i = 0; i < hi; i++) {
        if (i % 2 == 0)
          cout << "a";
        else
          cout << "b";
      }
      int k = 0;
      for (int j = i; j < n; j++) {
        k++;
        printf("%c", 'b' + k);
      }
      cout << endl;
    }
  }
}

### Prompt

Generate 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;
template <class T1>
void debug(T1 e) {
  cout << e << endl;
}
template <class T1, class T2>
void debug(T1 e1, T2 e2) {
  cout << e1 << "\t" << e2 << endl;
}
template <class T1, class T2, class T3>
void debug(T1 e1, T2 e2, T3 e3) {
  cout << e1 << "\t" << e2 << "\t" << e3 << endl;
}
template <class T1, class T2, class T3, class T4>
void debug(T1 e1, T2 e2, T3 e3, T4 e4) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << endl;
}
template <class T1, class T2, class T3, class T4, class T5>
void debug(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << "\t" << e5 << endl;
}
template <class T1, class T2, class T3, class T4, class T5, class T6>
void debug(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5, T6 e6) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << "\t" << e5 << "\t"
       << e6 << endl;
}
int main() {
  int n, k;
  while (cin >> n >> k) {
    if (k > n)
      cout << -1 << endl;
    else if (k == 1 && n > 1)
      cout << "-1\n";
    else {
      int i;
      int hi = n - (k - 2);
      if (hi > n) hi = n;
      for (i = 0; i < hi; i++) {
        if (i % 2 == 0)
          cout << "a";
        else
          cout << "b";
      }
      int k = 0;
      for (int j = i; j < n; j++) {
        k++;
        printf("%c", 'b' + k);
      }
      cout << endl;
    }
  }
}

```