#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  if ((n > 1 && k == 1) || (n < k)) {
    cout << -1;
    return 0;
  }
  string answer;
  answer.resize(n, 'a');
  for (int i = 1; i < n; i += 2) answer[i] = 'b';
  for (int i = 0; i < k - 2; i++) answer[n - k + 2 + i] = 'c' + i;
  cout << answer << endl;
  return 0;
}
