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