#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int M = 110;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double Pi = 2 * acos(0.0);
inline int max(int a, int b) { return a > b ? a : b; }
inline int min(int a, int b) { return a < b ? a : b; }
inline int bit(int x) { return 1 << x; }
inline int abs(int x) { return x > 0 ? x : (-x); }
int a, b;
int main() {
  cin >> a >> b;
  if (b > a || b > 26 || (b == 1 && a > 1))
    cout << -1;
  else {
    int t = a - b;
    if (t % 2 == 0) {
      for (int i = 0; i < t; i += 2) {
        cout << "ab";
      }
      for (int i = 0; i < b; i++) {
        putchar('a' + i);
      }
    } else {
      for (int i = 0; i < t / 2; i++) {
        cout << "ab";
      }
      if (b == 2)
        cout << "aba";
      else {
        printf("abac");
        for (int i = 3; i < b; i++) {
          putchar('a' + i);
        }
      }
    }
  }
  return 0;
}

### Prompt

Your task is to create 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 = 0x3f3f3f3f;
const int M = 110;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double Pi = 2 * acos(0.0);
inline int max(int a, int b) { return a > b ? a : b; }
inline int min(int a, int b) { return a < b ? a : b; }
inline int bit(int x) { return 1 << x; }
inline int abs(int x) { return x > 0 ? x : (-x); }
int a, b;
int main() {
  cin >> a >> b;
  if (b > a || b > 26 || (b == 1 && a > 1))
    cout << -1;
  else {
    int t = a - b;
    if (t % 2 == 0) {
      for (int i = 0; i < t; i += 2) {
        cout << "ab";
      }
      for (int i = 0; i < b; i++) {
        putchar('a' + i);
      }
    } else {
      for (int i = 0; i < t / 2; i++) {
        cout << "ab";
      }
      if (b == 2)
        cout << "aba";
      else {
        printf("abac");
        for (int i = 3; i < b; i++) {
          putchar('a' + i);
        }
      }
    }
  }
  return 0;
}

```