#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  if (n < k || (n > 1 && k == 1)) {
    cout << -1 << endl;
    return 0;
  }
  if (n <= 2) {
    n == 1 ? cout << "a" << endl : cout << "ab" << endl;
    return 0;
  }
  char *str = new char[n + 1];
  str[n] = '\0';
  int pos = 0;
  int prev = 1;
  char used = 'a' - 1;
  while (n - pos > k - 2) {
    if (prev == 1) {
      str[pos] = 'a';
      prev = 0;
    } else {
      str[pos] = 'b';
      prev = 1;
    }
    if (used < str[pos]) {
      used = str[pos];
    }
    pos++;
  }
  while (pos != n) {
    used++;
    str[pos] = used;
    pos++;
  }
  cout << str << endl;
  return 0;
}
