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