#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  if (k > n) {
    cout << "-1";
    return 0;
  }
  if (k == 1 && n == 1) {
    cout << "a";
    return 0;
  }
  if (k == 1) {
    cout << "-1";
    return 0;
  }
  for (int i = 1; i <= n - (k - 2); i++) {
    if (i % 2)
      cout << "a";
    else
      cout << "b";
  }
  char bb = 'c';
  for (int i = n - (k - 2) + 1; i <= n; i++) {
    cout << bb;
    bb++;
  }
  return 0;
}
