#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
const double eps = 1e-11;
const int mod = 1e9 + 7;
template <class T>
inline void readInt(T &n) {
  n = 0;
  T ch = getchar();
  int sign = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') sign = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    n = (n << 3) + (n << 1) + ch - '0', ch = getchar();
  n = n * sign;
}
int main() {
  string ans;
  int n, k, flag = 0;
  cin >> n;
  cin >> k;
  if ((k == 1 && n > 1) || (k > n)) flag = 1;
  if (flag)
    cout << "-1" << endl;
  else {
    string s = "abcdefghijklmnopqrstuvwxyz";
    if (k == 1)
      cout << "a";
    else {
      for (int i = 0; i < n - k + 2; i++) {
        if (i % 2 == 0)
          cout << "a";
        else
          cout << "b";
      }
      for (int i = 2; i < k; i++) printf("%c", 'a' + i);
    }
  }
}

### 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 double pi = acos(-1.0);
const double eps = 1e-11;
const int mod = 1e9 + 7;
template <class T>
inline void readInt(T &n) {
  n = 0;
  T ch = getchar();
  int sign = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') sign = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    n = (n << 3) + (n << 1) + ch - '0', ch = getchar();
  n = n * sign;
}
int main() {
  string ans;
  int n, k, flag = 0;
  cin >> n;
  cin >> k;
  if ((k == 1 && n > 1) || (k > n)) flag = 1;
  if (flag)
    cout << "-1" << endl;
  else {
    string s = "abcdefghijklmnopqrstuvwxyz";
    if (k == 1)
      cout << "a";
    else {
      for (int i = 0; i < n - k + 2; i++) {
        if (i % 2 == 0)
          cout << "a";
        else
          cout << "b";
      }
      for (int i = 2; i < k; i++) printf("%c", 'a' + i);
    }
  }
}

```