#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  int k;
  cin >> k;
  k--;
  string s(n, '.');
  int cnt[3]{};
  auto dfs = [&](auto&& self, int i) -> void {
    if (i == n) {
      if (cnt[0] && cnt[1] && cnt[2]) {
        if (k == 0) {
          cout << s << '\n';
          exit(0);
        }
        k--;
      }
      return;
    }
    rep(c, 3) {
      cnt[c]++;
      s[i] = 'A' + c;
      self(self, i + 1);
      cnt[c]--;
    }
  };
  dfs(dfs, 0);
  cout << -1 << '\n';
}