#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  int k;
  cin >> n >> k;
  if (k > n) {
    cout << -1;
    return 0;
  }
  if (n == 1) {
    cout << "a";
    return 0;
  }
  if (k == 1) {
    cout << -1;
    return 0;
  }
  vector<char> q(26);
  for (int i = 0; i < 26; i++) {
    q[i] = (char)(i + 97);
  }
  int w = n - k + 2;
  k -= 2;
  int o = 0;
  while (w > 0) {
    if (o % 2 == 0) {
      cout << "a";
    } else {
      cout << "b";
    }
    o++;
    w--;
  }
  for (int i = 2; i <= k + 1; i++) {
    cout << q[i];
  }
  return 0;
}
