#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
const string LN = "\n";
const double eps = 1e-9;
const double pi = acos(-1.0);
const int INF = (int)2e9 + 7;
const long long LINF = (long long)9e18 + 7;
bool insert(int k, int x1, int x2) { return (k >= x1 && k <= x2); }
const int MAXN = 60007;
const long long MOD = 1e9 + 7;
int n, k;
const bool is_testing = 0;
int main() {
  if (is_testing) {
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
  }
  cin >> n >> k;
  if (k == 1) {
    if (n == 1)
      cout << 'a';
    else
      cout << -1;
    exit(0);
  }
  if (k > n) {
    cout << -1;
    exit(0);
  }
  int d = k - 2;
  bool f = 1;
  for (int i = 0; i < n - d; i++) {
    if (f)
      cout << 'a';
    else
      cout << 'b';
    f = !f;
  }
  char c = 'c';
  for (int i = 0; i < d; i++) {
    cout << c;
    c++;
  }
}

### Prompt

Please create a solution in CPP 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>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
const string LN = "\n";
const double eps = 1e-9;
const double pi = acos(-1.0);
const int INF = (int)2e9 + 7;
const long long LINF = (long long)9e18 + 7;
bool insert(int k, int x1, int x2) { return (k >= x1 && k <= x2); }
const int MAXN = 60007;
const long long MOD = 1e9 + 7;
int n, k;
const bool is_testing = 0;
int main() {
  if (is_testing) {
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
  }
  cin >> n >> k;
  if (k == 1) {
    if (n == 1)
      cout << 'a';
    else
      cout << -1;
    exit(0);
  }
  if (k > n) {
    cout << -1;
    exit(0);
  }
  int d = k - 2;
  bool f = 1;
  for (int i = 0; i < n - d; i++) {
    if (f)
      cout << 'a';
    else
      cout << 'b';
    f = !f;
  }
  char c = 'c';
  for (int i = 0; i < d; i++) {
    cout << c;
    c++;
  }
}

```