#include <bits/stdc++.h>
using namespace std;
long long left[3009], right[3009];
int main() {
  std::ios::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  string s, str = "";
  long long n, i, k;
  cin >> n >> k;
  if (k > n || (n > 1 && k == 1))
    cout << -1;
  else if (n == 1)
    cout << "a";
  else {
    long long t = k, tt = n;
    t -= 2;
    for (i = 0; i < n; i++) str += ('a' + (i & 1));
    n--, k--;
    while (t--) str[n--] = ('a' + k), k--;
    for (i = 0; i < tt; i++) cout << str[i];
  }
  cout << "\n";
  return 0;
}
