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