#include <bits/stdc++.h>
using namespace std;
long n, k;
long ans[1000010];
int main() {
  cin >> n >> k;
  if ((k > n) || ((k <= n) && (k == 1) && (n != 1))) {
    cout << "-1" << endl;
    return 0;
  }
  if ((n == 1) && (k == 1)) {
    cout << "a" << endl;
    return 0;
  }
  if (!(n & 1)) {
    ans[1] = 97;
    ans[2] = 98;
    long c(96 + k);
    for (long i = n; i >= n - (k - 2) + 1; --i) ans[i] = c--;
    cout << "ab";
    for (long i = 1; i <= ((n - k) / 2); ++i) cout << "ab";
    if (k & 1) cout << "a";
    for (long i = (n - (k - 2) + 1); i <= n; ++i) cout << char(ans[i]);
    cout << endl;
  } else {
    ans[1] = 97;
    ans[2] = 98;
    long c(96 + k);
    for (long i = n; i >= n - (k - 2) + 1; --i) ans[i] = c--;
    cout << "ab";
    for (long i = 1; i <= ((n - k) / 2); ++i) cout << "ab";
    if (!(k & 1)) cout << "a";
    for (long i = (n - (k - 2) + 1); i <= n; ++i) cout << char(ans[i]);
    cout << endl;
  }
  return 0;
}

### Prompt

In cpp, your task is to solve 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;
long n, k;
long ans[1000010];
int main() {
  cin >> n >> k;
  if ((k > n) || ((k <= n) && (k == 1) && (n != 1))) {
    cout << "-1" << endl;
    return 0;
  }
  if ((n == 1) && (k == 1)) {
    cout << "a" << endl;
    return 0;
  }
  if (!(n & 1)) {
    ans[1] = 97;
    ans[2] = 98;
    long c(96 + k);
    for (long i = n; i >= n - (k - 2) + 1; --i) ans[i] = c--;
    cout << "ab";
    for (long i = 1; i <= ((n - k) / 2); ++i) cout << "ab";
    if (k & 1) cout << "a";
    for (long i = (n - (k - 2) + 1); i <= n; ++i) cout << char(ans[i]);
    cout << endl;
  } else {
    ans[1] = 97;
    ans[2] = 98;
    long c(96 + k);
    for (long i = n; i >= n - (k - 2) + 1; --i) ans[i] = c--;
    cout << "ab";
    for (long i = 1; i <= ((n - k) / 2); ++i) cout << "ab";
    if (!(k & 1)) cout << "a";
    for (long i = (n - (k - 2) + 1); i <= n; ++i) cout << char(ans[i]);
    cout << endl;
  }
  return 0;
}

```