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;
long long n, k;
string eh(int x) {
  string s = "";
  int p = 97;
  while (x != 0) {
    s += char(p);
    p++;
    x--;
  }
  return s;
}
string fu(long long x, int y) {
  string s = "";
  int p = 97 + y - 1;
  for (int i = 0; i < y - 2; i++) {
    s = char(p) + s;
    p--;
  }
  x -= (y - 2);
  string h = "";
  for (int i = 0; i < x; i++) {
    if (i % 2 == 0)
      h += 'a';
    else
      h += 'b';
  }
  return h + s;
}
int main() {
  cin >> n >> k;
  if (n == 1 && k == 1)
    cout << "a";
  else if (n < k || k < 2)
    cout << "-1";
  else if (n == k)
    cout << eh(k);
  else
    cout << fu(n, k);
  return 0;
}
