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