#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  string s = "abcdefghijklmnopqrstuvwxyz";
  cin >> n >> k;
  if (n < k || (k == 1 && n > 1))
    cout << "-1" << endl;
  else if (k == 1 && n == 1)
    cout << "a";
  else {
    if (((n - k) + 2) % 2 == 0) {
      for (int i = 0; i < ((n - k) + 2) / 2; i++) cout << "ab";
    } else {
      for (int i = 0; i < ((n - k) + 2) / 2; i++) {
        cout << "ab";
        if (i == (((n - k) + 2) / 2) - 1) cout << "a";
      }
    }
    for (int i = 0; i < n - ((n - k) + 2); i++) cout << s[i + 2];
  }
  return 0;
}
