#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
long long power(long long a, long long b) {
  long long ret = 1;
  while (b) {
    if (b & 1) ret *= a;
    a *= a;
    if (ret >= MOD) ret %= MOD;
    if (a >= MOD) a %= MOD;
    b >>= 1;
  }
  return ret;
}
long long inv(long long x) { return power(x, MOD - 2); }
const int N = 1e6 + 5;
int n, k;
char A[N], B[27];
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    cin >> n >> k;
    if (n < k) {
      cout << "-1\n";
      return 0;
    }
    if (k == 1 && n != 1) {
      cout << "-1";
      return 0;
    }
    for (int i = 0; i < 26; i++) B[i] = 'a' + i;
    int left = k - 2;
    for (int i = 1; i <= n - (k - 2); i += 2) {
      A[i] = 'a';
      A[i + 1] = 'b';
    }
    int cur = 2;
    for (int i = n - (k - 2) + 1; i <= n; i++) {
      A[i] = B[cur];
      cur++;
    }
    for (int i = 1; i <= n; i++) cout << 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 int MOD = 1e9 + 7;
long long power(long long a, long long b) {
  long long ret = 1;
  while (b) {
    if (b & 1) ret *= a;
    a *= a;
    if (ret >= MOD) ret %= MOD;
    if (a >= MOD) a %= MOD;
    b >>= 1;
  }
  return ret;
}
long long inv(long long x) { return power(x, MOD - 2); }
const int N = 1e6 + 5;
int n, k;
char A[N], B[27];
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    cin >> n >> k;
    if (n < k) {
      cout << "-1\n";
      return 0;
    }
    if (k == 1 && n != 1) {
      cout << "-1";
      return 0;
    }
    for (int i = 0; i < 26; i++) B[i] = 'a' + i;
    int left = k - 2;
    for (int i = 1; i <= n - (k - 2); i += 2) {
      A[i] = 'a';
      A[i + 1] = 'b';
    }
    int cur = 2;
    for (int i = n - (k - 2) + 1; i <= n; i++) {
      A[i] = B[cur];
      cur++;
    }
    for (int i = 1; i <= n; i++) cout << A[i];
  }
}

```