#include <bits/stdc++.h>
using namespace std;
int main() {
  long a, b;
  cin >> a >> b;
  if (a < b or (b == 1 and a > b))
    cout << "-1" << endl;
  else if (a == 1 and b == 1)
    cout << "a" << endl;
  else {
    long c;
    c = a - b + 2;
    for (int i = 0; i < c; i++) {
      if (i % 2 == 0)
        cout << "a";
      else
        cout << "b";
    }
    c = a - c;
    for (int j = 0; j < c; j++) cout << char(99 + j);
    cout << endl;
  }
  return 0;
}
