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