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;
#pragma comment(linker, "/STACK:64000000")
int n, m, i, j, k, u, v, w;
const int maxn = 51;
const int maxm = 100100;
const int inf = 1e9 + 7;
const int mod = 1e9 + 7;
string ar;
void init() { cin >> n >> k; }
void solve() {
  if (k == 1) {
    if (n == 1) {
      cout << 'a';
    } else {
      cout << -1;
    }
    return;
  }
  if (k > n) {
    cout << -1;
    return;
  }
  -- --k;
  n -= k;
  for (i = 0; i < n / 2; ++i) {
    ar.push_back('a');
    ar.push_back('b');
  }
  if (n % 2) {
    ar.push_back('a');
  }
  for (i = 0; i < k; ++i) {
    ar.push_back((char)(i + 2) + 'a');
  }
}
void out() { cout << ar; }
int main() {
  init();
  solve();
  out();
  return 0;
}
