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
#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m;
  cin >> n >> m;
  if (m > n || (m == 1 && n > 1))
    cout << -1 << "\n";
  else if (m == 1 && n == 1) {
    string lo = "a";
    cout << lo << "\n";
  } else {
    string temp = "";
    long long d = n - (m - 2);
    int flag = 0;
    while (d--) {
      if (flag == 0) {
        cout << 'a';
        flag = 1;
      } else {
        cout << 'b';
        flag = 0;
      }
    }
    for (int i = 1; i <= m - 2; i++) {
      cout << char(98 + i);
    }
    cout << temp << "\n";
  }
}
